Grear vendors

You asked:
"What trans mainshaft does the gear venders require in an A body. The website says to change to a B body length mainshaft. …. Unless I’m reading that incorrectly."
This is correct.
I had to procure a long shaft, which is commonly called the B-Body output shaft.

At the time I got mine, 2004ish, the ONLY way it came, was with the B-body length adapter-casting, which was good, cuz to fit the GVod into an A-body with a Short tail trans adapter, would have required opening up the floor. As it is with the longtail, now several inches rearward, it is still daymn tight.

After the GVod was installed in my 68 Barracuda, the driveshaft needed to be shortened by however long the unit was, plus the difference between the A and B body tails. This brought it down to IIRC under/around 3 ft.
The GVod has the LARGE spline output same as the A727. and mine came with a 1350 yoke; so from that point on, you can use any driveshaft-core, cuz yur gonna have to cut it anyway.
In my case, I just sent the two yokes and told the shop I needed it be able to withstand 450 hp. I sent along an old driveshaft for them to cut up. But after cutting the one end off, I guess they decided it was unacceptable cuz I got it back that way, lol.
It cost me a lotta money, but it's never given me a lick of trouble.