If the U-joints and slip joint are in good shape, properly timed and the angles match, there shouldn't be any vibration to speak of. In that case, running along at 55 MPH will be a bit noisy, mostly because of the fan, but not unreasonable. Also, those trucks came with 32 or 33 inch tall tires in 7.00 or 7:50x17 size. The 7.50 size is still available from Coker and maybe others but not the 7.00 AFAIK.
By the last years of the task force trucks 19.5 wheels were offered. There are a couple of radial sizes of these available that might do for you. And you may be able to find steelies with hubcap clips. They will get you to 55 MPH at a more reasonable RPM, 3300 @ 60MPH as I recall. I"m sure there are many 4.57 punkins around that will drop your RPM by around 350 RPM at highway speed. They are found standard in 3/4 tons.
I would avoid a T5 in a 1-ton, especially for towing. There were Spicer 5-speed OD trans offered in the big trucks that have the correct bolt pattern. I have seen one installed behind a 302 GMC in an AD GMC PU. Except for the floor pan it was a bolt-in. Clark was also offered but I believe parts are NLA for those.
