Eric, I applaud your commitment to the manual transmission. I can't imagine my 65 Chevelle any other way.
A good friend of mine, Andy Starr just completed Drag Week with his 56 Chevy, running G Force trans. He uses a Dragon Claw Clutch and an adjustable clutch yammer but frequently kills ring gears in his Dana 60. Stepping up to a set-up like that is expensive and likely will result in a longer trip down a road of breaking more parts and engineering the weak link(s) out of the who car.
Selfishly, I would just like to see a bunch of success, fun & enjoyment for you- in the way of low E.T.s, exceeding your own expextations and minimum carnage on parts. The TH400 offers a quick and direct path to that kind of fun. Might be a harder choice to make than it appears, which I can relate to. Between the time already into the 4 speed and a bit of ego on figuring out how to get the manual to meet your goals- it isn't such an easy choice to make.
This is a bit long, but I thought about your dilemma several times while reading Andy's blog.
http://hilborninjection.com/drag-week-2016/