Gotta chime in...
Any transmission that is not an overdrive is 1:1 in high gear, be it automatic or manual, so unless you're going 700R4 or a 5 speed manual, you won't be changing your RPM any. Since you just clued us in that it has a '74 350 in it, then does it have the mounts at the front by the timing chain cover, or did they make the mount brackets to use the '74 engine mount location? Original would have the two rear mounts at the bell housing. If you use ANY late model trans, you must go with mount brackets to fit the side mounts for the '74 and make a new cross member to mount the transmission tail shaft housing. For the time and effort you'll spend, you might be better off trying to locate a "Brownie" auxiliary transmission, ( 5831 B) or a Gear Vendors unit. Brownies are a bit hard to find, but they're out there. Expect to spend between 2 and 3K for either one.
As to 70 MPH speeds, just remember that there weren't any interstates back them. Most road conditions prevented you from exceeding 50 - 60 MPH, and that trucks brakes were designed accordingly!
