65 barracuda driveshaft length / transmission question

Hey folks!

I am restoring a 65 barracuda, V8 auto. Originally the driveshaft had a flange connection to the transmission instead of a U joint.

Well, to make things complicated I do not have the original transmission, but I do have a later model 1978 904 auto transmission. I should also mention I have upgraded the rear end to a 8.25in A body.

Once I get the transmission in, I obviously will measure the driveshaft length, but I am wondering if anyone has done this combo before and has any advice on if there is an existing driveshaft length I can grab from a junkyard? Or is it likely I will need a custom length?

Also, has anyone ever put a 904 1978 trans in a early A-body? Any gotchas?

Thanks!