Here's a bit of info which might be of interest. I'm also involved in a similar project building a 1991 164s engine for for my 82 gtv6.
I have in my parts stash two 2.5 Milano engines, one is early production the other a late production engine, the crankshafts are different in the rear seal/flywheel flange area. The early engine is the same as a gtv6 crank, the
later engine is the same as a 164 crankshaft in that area. The later crank shafts are shorter, so the late Milano flywheel has a longer flange where it mates to the crankshaft. So if you use the LATE Milano flywheel you don't need a spacer, you also need the late Milano flywheel bolts.
The Milano flywheel is pretty heavy so I lightened mine in a lathe, cutting it down to a bit less than the weight of the 164 flywheel.
As you may, are may not know, the 164 block is approx 4mm/.160 in shorter than the gtv6/Milano block, but a spacer plate of 1/8 in. (.125) aluminum
(.160 aluminum plate is hard to come by) between the block and the bellhousing recovers most of this distance.
I could probably send you some pics if you need them.