[mechanicalman]

700R4 with TCC will get you low RPM's on the highway/better gas mileage but be warned engine efficiency can suffer if the cam is not in sync with the compression of the engine and the rear end gear ratio. Better to settle on rear end gear and trans to see what your cruise RPM is then select a cam to match those RPMs and your compression ratio.

I'd shoot for 9.5 compression but would recommend vortec heads/intake manifold and a GM cam like the HO350/330 HP hydraulic cam 212-222-112.

Headers and free flowing exhaust are the best thing you can do for it, however right now with the 3 OTT you likely have a bell housing cross-member that may limit your header selection in a negative way. Maybe start with "what is your rear axle ratio?" and install the appropriate automatic trans, 700R4 or T350 or T350C with the appropriate trans cross-member keeping in mind when you obtain a cross-member you might want to consider exhaust pipe clearance. With the long wheelbase you could probably add an "X" pipe or at least a cross-over pipe, also stick with 2.5" pipes all the way back.

Hard to predict what mileage to expect not knowing rear gear ratio, cruise RPM, intended transmission, etc.

Fuel injection is going to cost you. A well tuned Q-jet or AFB will do a great job but also critical is an appropriate ignition curve on the old distributor, and I'd advise you to get a new HEI as they are so cost-effective now on e-bay then a curve kit if necessary plus an adjustable vacuum advance.