I think I understand the confussion. The '50s
'sixes' bellhousing bolted to the engine that was narrow at the back the same as the front.
The trans to bell housing didn't matter 6 or 8. The '50s and '60 V8s bolted the same to engine, and trans patterns were the same.
However, Chevy in the '60s changed the '6'
block in the rear with the wide flange so the bell housings were the same pattern to the engine as the V8. The same common trans to bell housing stayed the same.
If you want an six/at in an old truck, I think a nice set-up is the 250/at out of a late '60s, early '70s Nova/Camaro. Not too heavy, plenty of pep, quiet, little work needed on brakes and steering. They have
(I think) 5 main bearing so they don't shake
and can take the higher RPMs. You can paint them to look old like the original.
