I may have been off on the speed it will shift at ( I was going by my truck with a 3.54 rear gear and a 3 something first that shifts at 6700 RPM going about 35mph-and I do that about 20-30x a day, every day), but you still are not considering how far the RPMs will drop when going from 1st to second, and how that compares to your torque curve. That is the shift recovery point that I was talking about. As for the TH350, I will agree with you there as I HATE all automatic transmissions. My car only has one because I don't yet have the cash to put in a manual. I tend to prefer close ratio boxes because of the whole shift recovery point thing as it lets me stay in the power curve when driving aggressively. Plus, transmissions with taller first gear ratios tend to be stronger than those with shorter (numerically higher) ones. My ultimate transmission would be a six speed manual with a 1:1 6th and a rear gear set up with around a 2.14:1 final drive ratio. Then I could set up the other gears to be where they would be with a shorter rear gear. This is actually the strongest setup as overdrive stresses the drivetrain, and shorter rear gears compromise strength with a smaller pinion diameter and a smaller cross section of gear tooth engagement than taller ones do. Plus, it gives you the mathematical equivalent of the overdrive transmission so you lose nothing in terms of economy.
However, we live in the real world and most of us can't afford to have a custom rear end and transmission set up for our cars. So what would I do? If you want a cheap manual, go for a T-5. The 1st is still taller than the TH350's, but not extremely tall. It also was produced in a wide variety of cases and applications, so you can tailor it to suit your needs if necessary. However, a 88-92 Camaro V8 application would probably be the best choice (those years are the World Class T-5's in Camaros). It is the strongest of these weak transmissions, and has a reasonable set of gears in it. It is probably every bit as strong as the T176 but comes in a 2WD passenger car version. If you have a strong drag car though, neither of these transmissions will hold up for long with slicks. In a drag car, I would consider the T56 6 speed manual, but understand that making it fit is a real PITA. It is stronger than ANY T-5 and most 80's transmissions of any sort, but it is larger and the price is much higher. The other logical option is a Richmond Super T-10. In a drag car, you don't need overdrive, so you can save some cash there. You can also get the gearing of your choice if you buy it new or modify it.
Whatever you choose to do, start with a 2WD transmission. The amount of things you will need to change to use a 4WD one in a 2WD car makes it very difficult and expensive. They include (but are not limited to): the tailshaft, output shaft, shifter, and sometimes the shift rail too. Also, don't just look at gearing. Look at strength too. Most good transmissions that can easily fit in a car have an aftermarket following. If you find one that does not, ask why. Lastly, I am not saying NOT to think outside the box and consider things that no one else has done. I think outside the box all the time and investigate the odd and quirky to see what it can do for my needs (I dig the Zaporozhetz 968M, so I am by no means conventional!). However, you should realize that it is exceedingly rare to discover a 30 year old part that will work well with a SBC but that no one else is using. These engines are the guinea pigs of many smart and stupid people alike, and if it works it usually can be found documented somewhere. So, try new things, but don't waste your money either.
