Zip:
You are right about Muncie flex. However the aftermarket makes a cast iron midplate that reduces it significantly- at least the ones I have thrown together for guys seem to be OK.
I put them together on the side for guys and use the cast iron midplate, the bigger front bearing, grooved bushing on number 1, and torq lock sliders as upgrades. I have a friend thrashing one right now in a high horse 65 Chevelle.
Also if you are worried about leakage where the countershaft exits the front of the case, it can be bushed there (though I use a little permatex hi-tack and it cleans up any bit of slop in the press-fit)
B, if you decide to keep the Muncie, I can rebuild it for you with the upgrades pretty reasonably- Just a fall back if you dont do the 5-6 speed upgrade.
S