Oh man, that'd be sick.
MX73s come with 5Ms, so I assume it'd be similar to a 7M swap in an MK2 Supra. This swap is actually done quite often (especially in the past), I think the hardest part would be the manual conversion, if it isn't already manual. I happen to know that they stopped production of manual Cressidas all together at a certain point in that generation, so that may be a problem if your car isn't already manual. However, the motor swap itself is relatively simple. The 7M bolts right in no problem. Also, if you elected to keep your automatic transmission (if your car happens to be automatic of course) the A340E is shared between the 5M and the 7M, so you can retain the stock transmission.