Overdrive tranny in an A-body

AJ/FormS said
This swap has been successfully done many times. There is now even a brand new cross-member being repoped (USCarTool) so you don't have to fab one yourself. But no matter what, the floor has to be cut open, and the T-bar crossmember raised and reconstructed.

Edit;
no matter what; the floor should, (that's should), be cut open to raise the driveline centerline back up to it's factory height. If this is not done, the front U-joint angle becomes near zero. And so then, to restore the factory freedom-from-vibration angles, and the proper U-joint cycling, the rear pinion angle has to be adjusted...... but, unless you raise the rear of the car significantly, it can never properly equalize the angles. So, I suspect that under power, you will have U-joint problems.
I don't know this to be true.
But;
I bolted a GVod onto my long-tail A833, and ran into this exact same thing; an elusive,impossible to eradicate vibration, traced to the U-joint angles.
My car is lowered about 2 inches and leveled front to rear,and I only lowered my tail mount about 1/4 inch, compared to what looks like several times that, with the USCarTool dropped Crossmember.
So Plumkrazee70 is right that when using that mount, you don't "have to" cut the floor.........
But if I was doing it, and I probably never will, I would raise the back right thru the floor cuz that U-joint angle is already from the factory, not enough for a performance driving with a lowered ride-height.
I mean, to me, the benefits of running that overdrive are well worth the work to make it vibration-free, with 3.91s to 4.10s.
Now, If you run 2.76s like Trailbeast does, this slows the driveshaft to ~67% compared to 4.10s, and if you run 65mph instead of 80, that is a further reduction in driveshaft rpm to 81%. Taken together that should reduce the vibration very significantly, to the point that you might be able to get away with the USCarTool dropped crossmember.
But that won't solve your U-joint life problem. No biggie; I just put a 1350 joint on there.
And it won't solve your vibration when the axle winds up under power and takes the front angle from zero to slightly nose down.... to nose up; no problem, I just lowered the pinion some more.
I still have some cruise vibration left.