There are long and short TH-400's. The driveshaft dimensions for them are drastically different. I've never tried to put a long tailshaft TH-400 in a FGF, but I assume it can be done.
You might need a different (longer?) speedometer cable.
I have always gotten whatever car I'm working on put together (as far as I can, anyway), tried the driveshaft I have, and measured how much it needs to be shortened. (So far, all my driveshafts have been too long.)
Take the driveshaft to a shop, tell them how much you need cut out, and they can do it. You might as well have new u-joints put in it at the same time.
It will also need to be balanced.
Good luck!
Wayne.
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