Did you have a manual transmission before the first TH350? Did it only run about 50mph before you changed over to the first TH350? If so then you may not have a 3:90 ratio after all. Manual transmission or automatic will have 1:1 in high gear unless it has an overdrive.

So the TH350 wouldn't get you down the road any faster than the manual 3 or 4 speed transmission that may have originally been in your truck to start with. Especially if you have 4:11 or other lower geared rear axle. The only two ways to lower rpms for higher road speeds is either with an over drive transmission or better gearing in the rear end. Or both.