First I'd do a compression check on all the cylinders to get an idea of where you are. If all is good, early 273's have decent compression so your stock pistons should be good. I'd swap in a cam, lifters, and matching springs. Use the newer Viton positive valve stem seals while the springs are off. Exhaust manifolds are all the same on 64 - 66 A bodies. Get custom duals with turbo mufflers or the original Performance exhaust was 2.50" single with two straight through mufflers in line. Get a 4 barrel manifold, see above, an original Carter AFB, or a 500 cfm Edelbrock with an adapter plate. The best milage Carb would be a TQ, ThermoQuad, and an adapter plate. People will tell you that it is too big, but we used to run them all the time on 273's. Get the earliest you can lay your hands on. They have the least smog complications and have a ported vacuum for your vacuum advance. You just adjust the secondary air valve till it does not bog when you floor it. Last I'd add a performance electronic distributer. Set your timing at 35 degrees total without the vacuum advance. Make sure the fuel system is clean and replace fuel filters. Now in short order you will need to get a 10" performance clutch, pressure plate and throw out bearing , from somewhere like Brewers Performance, since your stock one will not last long. Get your brakes and suspension up to snuff. Stock is fine but you will probably enjoy a front sway bar and 15" x 6" wheels and sticky low profile tires. You can go bigger but it is hard to beat the efficiency of a 273. A Performance 273 with a 4 speed is pretty sweet "all around" A body. Enjoy.
