Hypothetical build questions…

If I was picking between the 2 cams you listed, I would hands down pick the MP '761' cam. I have that cam in a much heavier 71 Challenger with a mild 360/727/3.23's, and it will get 20 mpg on the highway. Either run a regular 833 four speed with 3.23 gears, or the 833OD with 3.55 gears. You will have to baby the OD trans probably, if the 340 has at least stock amount or higher HP.
Noted in other posts, any real power on 75-76 alum od trans, will shred the OD gear while in od