symptoms sound more like fuel than spark, insufficient delivery at higher rpms...

You need a pressure regulator for a weber. They want lower fuel pressures than the delivery of the stock pump. Don't remember the #'s...

This latter, however, is probably not your problem. You have either a bad fuel pump (go to Home Depot and buy a 30psi plumbing gauge and clamp it to the pump output line and see what the deadhead pressure is), a clogged filter, a blocked pickup in the tank (old fuel varnished it up). If the truck sat for any appreciable length of time, the fuel goes bad and gums stuff up...