The best person for this would be a member called "Intelligeorge", if you send him a pm im pretty sure he could answer it, hes pretty cluey with GM's.
I think the simplest way to answer your question is to ask if the rear transmission oil seal will sit at the same point with the new set up. If it does it will point towards the same length tailshaft, but have you checked if the m21 yoke will fit into the turbo400 (i dont think so) and if a turbo 400 yoke could be fitted to the standard tail shaft (are the universal joints the same size).
Now the hard part...how i use to calculate tailshaft lengths.
Stock diff; Take the springs off or load it with sand bags till the diff is on its bump stops while the car is on diff axle stands so you can get under it.
Measure the diff flange (if flat) or the center of the uni joint (if curved diff flange with u bolts) to the transmission oil seal.
Place the turbo400 yoke into the tranny all the way in and pull it out by about 3-4mm for non leaf spring setups or 6-8mm for leaf springs and mark where the oil seal sits with a marker.
You now know the length of your tail shaft from the flange or center of the universal joint (depending on the diff flange you have) to the marking pen mark on the yoke that goes into the tranny.
Now jack your car up till the diff is hanging off its shockers and remeasure.
Make sure the extra length leaves enough yoke in the tranny so the tailshaft cant fall out if you get airborne.
If setting up a aftermarket diff yourself make sure you setup the geometry correctly.
The diff pinion should be pointing at the tranny output when the car is sitting on the ground with a driver and fuel in the tank if its non leaf spring.
Leaf spring setups require the diff pinion to look a little lower to allow for leaf spring windup on hard launches, and a little less if your running tramp bars.
Check a mid 70's statesmans drive shaft, i think a mate had on with a 350chev and turbo 400, im not sure if they used the same wheel bases though.
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