You also need to consider your final gear ratio in low gear. It's going to take some heavy clutch-slipping to get it moving from a dead stop with 3.08's and a Muncie.
When I changed my Nova to 3.08's, it was real uncomfortable crossing a busy highway from a stop sign, so much so that I ended up swapping the original trans, which had a 2.54:1 low gear, to one with a 3.11 low gear. That's in a 3200# car with a nearly stock 350 and a heavy flywheel. If your Muncie's an M-21 or M-22, with the 2.20 low gear, it will be even worse.
Might be worth the trouble to see if you can find a used center section with 3.08 gears already in it, and stick it in your housing just to see how you like it. '55-'64 Chevy cars & '55-'62 Corvettes use the same center section.