If you put in3.55 rear end gears and run 28.5" to 30" tires on the back you'l be good at 60-65 mph. You'll need to change your speedo gear.
The Tuuckstill OD unit is a pipe dream. Like I say I have only seen one complete unit and any one found would have to be gone through. There is a lot of linkage involved. There is a company that makes a unit designed for Model As. They may make one for Chevys.
With any of the OD transmissions I know of you'll be better off going to an open drive shaft.
Something else to consider if any sustained highway speeds are planned the front wheel caged ball bearings should be replace with the tapered roller kits offered by after market venders.
