Although it isn't easy, you CAN replace your 3.0L SHO engine with a 3.2L out of a 93+ Automatic SHO. In addition, the SHOShop (www.shoshop.com) provides higher displacement SHO engine upgrades and complete long blocks for direct replacement.
You also have a little room to bore and stroke the 3.0 motor. The SHOShop has created blocks as large as 3.6L (but are they reliable?). If you decide to punch your 3.0 out to 3.2 and you want to use the stock 3.2 pistons, be aware that the wrist pin diameters are not the same. The 3.2 pins are a hair larger to take the load from the larger piston. All of which means you'll also need different wrist pins and connecting rods.
Thanks to Tim Dahm for originally discovering the ability to swap the 3.2L motor into an MTX SHO. It was previously thought to be impossible, but the procedure has now been repeated several times. The use of 3.0L intake cams may help produce a few extra HP at high RPMs when installed on a 3.2L motor.

3.2L MTX Swap Info Thanks to Tim Dahm:

Part 1

I picked up a low mileage 93 or 94 3.2 ATX engine assembly in hopes of fitting to a 90 MTX.
I've started the disassembly and comparison process:

3.2 longblock with mtx flywheel bolted up: Click Here

3.2 longblock with mtx-4 bolted up: Click Here

3.2 with 3.0 ps pump/motor mount bracket bolted on: Click Here

3.2 with 3.0 alt/ac/front motor mount bracket bolted on: Click Here
note the two upper holes for the lift bracket, the 3.2 head is not drilled or tapped for the 3.0 lift bracket.

coil mount details, 3.0 head on left, 3.2 head has mounting boss for 3.0 coil bracket but not drilled or tapped. Click Here

front head lift bracket holes not present on 3.2 head, 3.0 head on left: Click Here

front head accessory side, 3.0 head on right, 3.2 head has additional mounting boss with three tapped holes. This raised boss will interfere with 3.0 alternator, could possibly be milled off: Click Here

rear head accessory side, 3.0 head on left, no external differences found: Click Here

block accessory side, 3.0 block on right: Click Here 3.2 block has extra tapped holes, but all necessary holes for 3.0 timing belt tensioner are drilled and tapped in the 3.2 block.

block 3.2: Click Here

block 3.0: Click Here

In summary:

It appears the stock 3.2 short block will work as is, the 3.2 rear head is identical externally, the 3.2 front head will require modification, all other bolt on parts must be from a 3.0 (oil pump/pan, timing belt tensioner, timing covers, all accessory brackets, motormounts, exhaust manifolds, intake assembly, water pump etc)

Next up I will pull the front 3.2 head and compare combustion chamber design/volume with a 3.0 head.

Part 2:

I've gotten a bit further into my 3.2 vs 3.0 comparison. I noted in my previous post that the 3.2 front head needs modification to use in a mtx car.

front 3.2 head before modification: Click Here

front 3.2 head after modification: Click Here

chunk of front 3.2 head cut out with sawzall (I know- hack job) Click Here

with the front head modified, the 3.0 alternator/ ac compressor/ motor mount bracket all fit correctly: Click Here

will have to drill and tap a hole in the front 3.2 head for the lift bracket: Click Here

the long alternator bolt will have to be shortened 3/8- 1/2": Click Here

front view of the 3.2 longblock with 3.0 oil pan, oil pump, tensioner, and waterpump, and upper timing cover backing plate: Click Here

one of the bolt holes in the timing cover backing plate does not line up with the front 3.2 head, I will leave it out- the other 8 bolts will hold it just fine: Click Here

Summary:
So far so good, no major problems yet, looks like I can even use the 3.2 heads with the above modification. For those who didn't see my previous post, I already confirmed that the mtx flywheel bolts right up to the 3.2 crank, the mtx-4 transaxle bolts right up to the 3.2 block, and both front and rear 3.0 motor mount brackets bolt up to the 3.2 block. The timing belt tensioner stud and pin must be transfered from a 3.0 block-
the proper holes are present in the 3.2 block.

Next:
I will finish the accessory end of the motor, install cam pulleys, timing belt, timing covers, and accessory belt drives, all will be from a 3.0 mtx motor except cam pulleys- 3.0 and 3.2 are identical. I will not be pulling the front head to compare combustion chamber design and volumes between 3.2 and 3.0 heads, as there seems to be no need to use the 3.0 heads. Also need to check if different 3.2 coil mounting bracket will work ok in mtx car, if not will have to drill and tap a couple more holes in the front head.

Part 3:

More info on swapping a factory 3.2 motor into a mtx SHO:

3.0 coil/bracket mounted on 3.2 long block, note two mounting holes with black circles drawn in them. These will need to be drilled and tapped in the rear of the front head: Click Here

or

it appears possible to just use the 3.2 coil/bracket on the 3.2 longblock in a mtx car, I don't think the angled mounting of the 3.2 coil/bracket will interfere with anything. Will require use of the 3.2 plug wires: Click Here

Here is a pic of the complete 3.0 accessory drive system installed on the 3.2 longblock: Click Here
The following 3.0 parts were used: oil pan, oil pump, timing belt tensioner pulley, stud, and pin, waterpump/pulley, crank pully, timing cover set, idler pulleys and brackets, ps pump/rear motor mount bracket, alternator/ac compressor/front motor mount bracket, alternator. Only the front 3.2 head required a modification (as detailed in previous post) to allow proper fit of the 3.0 alternator and bolting the engine lift bracket to the head.
Everything else bolted on with no modifications.

Front view of motor, 3.0 exhaust manifold shown: Click Here

Rear view of motor, 3.0 exhaust manifold shown: Click Here

Summary:

I am now 99.999% sure this 3.2 motor swap into a MTX car will work I will be 100% when it is installed and running. Still need to order a new 3.0
oil pump, stage 1 or 2 cams and install everything... it will be a while yet.

The only question remaining is... who said the factory 3.2 / mtx combo won't work??

Correction: I am now 100% sure the swap works as the 3.2 has been up and running in a 90 mtx for about a year with no problems... so was it worth it ? YES!!!
I have five SHOs and this one feels the most powerfull for street driving. The extra low end torque is what this car needs. I did install stage II cams and now a Performance plus Y-pipe and dynomax catback. Yet to be installed is a LPM/80mm maf/extrude honed intake runners/ big bore butterflys/ 68mm throttle body/ and sho shop can.... even with the stock intake setup this thing screams!

Here is a shot of the car the 3.2 is installed in:
Click Here