[jocko]

chewy, numbers below for a 29" rear tire. Note I only accounted for the overdrive in final gear for the GV unit (right chart in each row) - but remember you can split between gears with a GV unit. Bottom end accel remains the same as a stock TH400 with the depicted rear ratios, but you essentially get a 22% o/d for each of the 3 speeds - again, only o/d for 3rd is depicted for comparison.

So, my first car was a 57 Bel Air with a 68 327, Muncie 4-speed, and 3.55 rear end. Very similar to your current combo from the bottom end accel and top end cruising rpm perspectives. It's a good combo to be honest - but as you can see in the upper left chart, your current setup, you're slightly over 3000 rpm at 75 mph.

It's important to remember that the often unmentioned advantage of an overdrive trans, like the 700R4, is that it not only reduces your cruising rpm at highway speeds, but it also has a notably deeper first gear for bottom end accel improvement when compared to a TH350 or TH400 (note your final drive in a 700R4 with 3.54 rear would be 10.83:1 vs your current 8.78:1). BUT - your 55 mph cruising rpm is only 1579 rpm with a 700R4 and 3.54 combo, ok but not ideal - will border on lugging the engine. You're fine at 65 and 75 mph though.

Realistically speaking, with your tires, an overdrive trans would be optimized with a 3.73 rear gear (2nd row, center chart).

Your suggested 4.10 with GV unit is depicted in the bottom right chart. Not a bad combo at all, you get the bottom end acceleration oriented combo of a 4.10 rear while still enjoying relatively low (but not the lower sweet spot of a 700R4/3.73 combo) highway rpm.

Lots of things to consider for most bang for the buck approach though. Personally, I think you'd get the most bottom end accel and lowest (but not too low) highway rpm bang for the buck with the 700R4 and 3.73 combo. And that's probably less expensive than a 4.10 and Gear Vendors swap (or at least comparable in price). A 700R4 will be fine behind your 327.

Another option, and a fairly bulletproof one - IF you don't feel you need MORE bottom end accel than you currently have, would be to simply add a GV unit to your current setup. Top row, far right chart. Very good highway cruising rpm range - and you're only changing a single thing from your current setup, i.e. adding the GV.

Bottom row middle chart is what you'd want to avoid if you don't want to accidentally chirp the tires at every stoplight. Note the 12.55:1 final drive in 1st gear of the 700R4/4.10 combo.

Anyway, food for thought, hope this helps.