Getting Ready for the Magnum Swap. (help?)

I would give up a lot to have the A500 overdrive in there, and to be able to run 3.91s. This gets you a starter gear of 10.73, and if you have never run this much starter, I can't tell you how much off-the-line snap this brings to the party.
And all your shifts are closer together, keeping your engine on the pipe more of the time
and it still cruises with a final drive of 2.70 so 65~2200 with 27s . This means you can still run a 268 cam no problem, on the hiway.And the Magnum has enough cylinder pressure to run that 268, and the TC is loose enough with 3.91s that you barely notice the soft low-rpm.
I would drop a stock Magnum truck combo into an A in a heartbeat with 3.91s, if I was looking for drop-in . You will be smiling for a good long time. It's a great starter engine, and a pretty good builder, meaning you can add on to it.
Make sure the 3.91s have a SureGrip!