'64 or 65 A-body 4-speed 833 transmission

If that unit has the 3.09 low gear in it, guys;here is your chance to get your hiway buzzzzzzzzzzzzzzzzz down. This is 16% more 1st gear torque multiplication. 3.55s will take off like 1.16 x 3.55=4.12s
The entire gear-train will need to be swapped into your 2.66 case, along with the mainshaft from your 2.66, and your late-model rear extension. This is as easy a swap as it gets and you will never regret it. Everything else is straight-in swap stuff.
If you have a B/E, this stuff will also swap into your case/extension,with your mainshaft.
Line up guys!