1975 A833 od, any useful parts for non od boxes

Mr Pierce, if that tranny is the one I steered you towards,I'm so sorry that deal went south.
Both synchronizer are bolt-ins for any other box,and the forks can be made to fit as well,either with the late cover, or cut off as mentioned for the early cover.But none of the gear-train will interchange with anything but exactly the same other box. And the mainshaft only fits the O/D box,no other.There are at least two O/D ratios that I know of and the only one gear that they share is the low-gear.
On another note;
A small-cam 408 streeter,would love that box, with 3.55s.
This will get you a 10.97 starter gear, and a 2.52 hiway gear.And everything Blu said above is right.That's a hard combo to fight. Heck, even I'd leave you alone.
So if you got it cheap enough, you might want to hang on to it for a bit. Perhaps the bug will bite, to check it out at some time in the future.
BTW, I think that extension of yours carries a rear bearing. The replacement M/S you just scored does not have provision for that. IDK if the tail is long enough for this shaft, but I guess you will find out. If you can't make it work, give me a shout, and I will send you one that will, as a token of apology.