Hello, I have a 84 transam with a tired 305 and a beat 700r4. I am picking up a smashed car with a new (3K miles on it) olds 403 and a fresh 200r4 (BOP) trany. I know basicly the 403 is going to go in fine but the 200r4 is a unknown. Are the splines and tail shafts the same on the bop 200r4 vs the 700r4?

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.

You know... if you put an Olds 455 crank in that 403 block, you have 500 and some cubes? *hint hint*