with EFI, I say go for it. but with a carb, I'd say try to keep your cruise RPM above 2000 rpm, or run a "stock" cam with no lope, that is all done power wise, by 5000.

my big block was running a cam that was 1800-6000 rpm power band. I had a TKO with the .64 OD and 3.23 gears with 28" tires.

5th gear was useless unless I was going 70mph or faster, at 65 my RPMs were down at 1700-1800, and the engine was lugging and surging. I tolerated it for a year or so because I knew the whole time that I was planning on 4.56 gears in the rear. and once I did, everything was great! tons of low end, and yet I could still cruise the highway at 2400 rpm, smooth as glass at 70mph.

the TKO will be smaller, and with the .64 OD you don't really need 6 gears. in a 1/4 mile run, you probably will NOT be shifting beyond 4th gear. I feel like you really only need 4 gears for pulling, then a 5th gear OD for cruising.

with that TKO I was running, the 4.56s, 28" tires, the top of 4th gear for me was up around 110-115 mph. I don't need more gears beyond that!