Driveshaft should be about 45" c to c.
FSM calls for one at 44.99 IIRC.
My 71 Duster with a 904/8.75 was 48.90" and that trans is about 4" shorter than a 727, making the DS longer.
I turned codfish onto these guys. They'll build whatever you need up to about 50", with yoke, and any ends you want 1310, 1350, 7260, 7290. They are local to me, race circle track stuff IIRC, and do nice work.
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For a driveshaft, contact these guys. Speak to Joel.
ALLSTATE DRIVESHAFT 760-228-1127
They have an e-bay store. I got one from them, shipped for $250. NICE piece.
codfish