| Splines are the same. Length is not.
I assume you've done the research, and have found an exhaust system and motor mounts to put a short-deck Olds into one of these cars.... and you know that the starter and battery and fuel pump are on the opposite side of the car, etc.
The drive shaft is likely to be the least of your problems. If it doesn't fit, you measure how long it needs to be, and take it to the driveshaft shop and say "Make this driveshaft x inches long and put new U-joints in it"; a few hours and about $150 later, you're done. If you have to change the front yoke to one with a different spline, or you need some special adapter U-joint, it'll be a little more of course. Basically a cheap little no-brainer.
Those other things I suspect won't be quite so simple.
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"So many Mustangs, so little time..." | 
