72 stepvan - slow up hills.. rough idle

[Mgicm]

HI
Hoping for some advice on this 72 chevy stepvan i have. Motor is a 250 six and its been to no less than four mechanics. Problem is rough idle, and slow as heck when encountering even the slightest hill. I lose about 15mph going up a hill when traveling 60 mph. It has always seemed to be like this and nobody can figure it out. We have changed plugs, wires, cap, rotor a few times. Had three mechanics time it which I dont think any of them did right. Vacuum checked out ok so Im told.

The rough idle is odd. Its only when you start of from a stop or at gear change that it is sputtering or shaking. Once you get into the gear it seems to run well. Changed fuel filter 2x and pretty much the same. Changed gas, and gas grade with no effect.

I cant tell if it is a fuel issue (pump, carb, line, filter), vacuum issue, or more to do with timing/distributor. How would you diagnose or troubleshoot?

Sure appreciate it.

thanks GP.