Chevrolet Repair: 79 chevy truck stalling, carbarator, air leaks

Question
I have a 79 silverado that keeps dying.  It is worse at speeds over about 35, but will do it at a red light too.  We have pulled the fuel tank, cleaned it and put in new unit inside, new fuel pump, all filters, cleaned the fuel lines.  I asked several mechanics and the answer was always the carbarator.  OK, so I put a good rebuilt engine in it (for other reasons) and a good quality rebuilt Rodchester 2 barrel carb. on it.  This truck has two fuel tanks, but I only use the left one, it is the one we have completely redone, and the other one is empty and I taped the switch to the left.  The exaust system, including the cat converator, is new also.  When I picked it up from the engine place today (where they also put the rebuilt carb on) I almost didn't make it home.  It's doing the same thing, dies at ramdon, didn't really want to start again, and I couldn't go over 30.  What is left?  HELP!

Answer
Hello Trellen,
First check to see if you have a fuel line running too close to a part of the exhaust system, maybe causing a vaporlock. If not, check all the connections for air leaks, and maybe even bypass the tank selector valve in case it has an air leak.
Check the gas cap to see if a vacuum is forming in the tank, starving the fuel pump.
I might even stick an electric in-line fuel pump in the system temporarily, possibly even out of a separate container, just to isolate the problem.
Van