If you do indeed have a 5 speed with a .80 overdrive, it will go about 64 at 2100. If you want to go any faster, turn up the rpm's. They used to let buses go 2450 with the same engine you have. If you get any faster gears, you will never see high gear anyway. You must remember you have about half the horse power and about a third of the torque of todays engines and no aerodynamics. Spring brakes is the only way to park a truck the size of yours. Before spring brakes, we had to rely on blocks, etc. And the first spring brakes were junk too. It wasn't until the 70's that Anchorlock invented spring brakes that worked. If you want spring brakes, go to any heavy truck or trailer dealer and ask for a pair of 30/30 spring brakes and about 20 feet of Synflex tubing and some fittings. You will need a single spring push button for the dash. They became obsolete in the 80's, but they are still around. I probably have some in the shop I never threw away. Find an air supply, plumb it to the dash button and then to the spring brakes. You can T it at the axle housing. I prefer to put a rubber air brake house at the axle housing to give it more flexibility. In order to drive you push the button in. To park, you pull it out. If the air pressure drops below about 50 lbs. it will pop out and set the brakes. Spring brakes is the only way to park a truck the size you have.
