wtp over drive trans behind 340 in a 68 a body what can i do

So I really want a OD trans in my car
Is there a trans that will bolt up and clear tunnel that I can run
Gear vendors is just a tad high for me fig I could buy trans then save for a rebuild.

But I have no ideal what if any thing will fit help would be great
Just a mild mild 340 counsel car
Thanks