in my old 6bt/nv4500 truck with 33 inch tall tires at 2000rpm I was going 59 mph on my gps. and @ 22oo rpms i was at 65, 70 mph was almost 2400 rpms. I then switched to a set of 37 in tires and was able to cruise at 70 mph just barely over 2100 rpms, I found that was the best combination for me, for my use of the truck and even with 37's and about 5 inches of custom lift( 4 inch shackle flip on the rear and a 1 inch zero rate, and custom front 52 inch springs with a pack built by me and national spring) the truck could still hook up to my gooseneck trailer, it was actually still 1/2 inch lower at the tailgate than my 97 dodge 3500. It ran do the highway smooth and was a very capable truck both off road and on. I could shoot my self for parting with that truck, but I gave it to my nephew for a graduation gift.
Now back to why I was posting this for a 4bt a 700r4 could be used with its .7 to 1 od ratio, a nv 4500 has .73 to 1, usgears Od has a .80 to 1 ratio.
I know of a 4bt in a 67 chevy C10 2wd truck and they went with a 200r4 trans for the .67 od ratio 100mph in that truck i was told was around 2500 rpms
A 4bt with a 31 inch tire and just a th400 would be at 2500 rpms at 50 mph 33's would put that motor at just a hair over 2300 rpms. and 3000 rpm for 65mph with 33's now fuel economy would suffer but you can easily change the gov on a ve pump to fuel to 3200 rpm and both the 4bt and 6bt would survive and a 35 inch tire would lower the rpms as well.