Ignition problems in the wires can cause that, when you're putting a strain on it. Check the wires in the dark to look for bad wires.

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...

Current fuel line setup, from previous owner, is the following. Fuel line from tank to electric fuel pump along rail on driver's side that goes to filter and then to the mechanical fuel pump on engine. From there are two outlets, one return back to tank and the other to the carburator. With the one going from mechanical fuel pump to carburator, I added a pressure regulator when I installed the Weber (guidance from this site). Pressure there is regulated to just under 3psi.
If I have the electric in line fuel pump, do I really need to route the fuel through the mechanical pump. Could I just go tank -> electric pump -> filter -> fpr -> Weber?
What about the return line to tank? Or should I leave everything as is and just replace the mech. fuel pump?
Thanks

Ok, here's what I've done since my last post:
Since there was an electric fuel pump, I bypassed the mechanical fuel pump that the previous owner had connected. Now the fuel line goes from electric pump -> filter -> fpr ->carb.
FPR is showing a little less than 3psi; could that still be too high for the Weber?
Checked timing and it was about 7 BTDC; advanced to just under 10 BTDC to see if it helped. Not much difference; still bogs down in 4th when giving more than half throttle.
While engine not running, pulled negative pressure on vacuum advance line and the pressure held.
Weber 32/36 has only been on a couple of months and I changed the fuel filter around the same time.
Any suggestions?? Could it be carb related. I see posts on here where people have changed out the jets, but my knowledge about carbs is very limited, so I don't know if that is a direction to explore.
Any additional help is appreciated.
Thanks