I don't know, I usually start at the front and work my way back.
1- Is the fuel bowl filling.?
IT SEEMS NOT

2- Is the filter plugged and in correctly?
NOT PLUGGED, CLEAN, IN CORRECTLY

3- Is the line from pump to carb good?
THE LINE FROM PUMP TO CARB IS NEW

4- Pump has proper fuel pressure and delivering fuel?
etc.
THIS IS BECOMING THE MAIN CONCERN. WHEN TESTED IT HAD GOOD PSI. BUT when connected to the Ext tank, all new hoses to mechanical pump, the fuel seems to not be filling the carb bowl. Further testing is proceeding.

It takes fuel in the carb to make it run. (Your 5 gal can and electric pump) If it won't run you can stop right there and fix your problem. Or, if it works fine you move back one step. Keep doing that and eventually you will find your problem. Working from the back forward will eventually work but it's much more positive working from the front back.
The OEM fuel tanks and hoses up to the sol valves are plugged and now, not in use.
I have an 18gal fuel tank on the rear rack, all new hoses to the front to the Mechanical pump. Idled good but when doing the test drive, the bowl lost level. there is nothing it seems that can do this except the mech fuel pump. will add an electric fuel pump in line to add to the upcoming test run. You are right. the problem will be found.

It may still be ignition. 74 is points and condenser. When it warms up the condenser could be failing or the hold down screw could be loose unless, you've converted to an electronic ign then it could be a bad unit.
-The ign was changed over to electronic when the tanks were dropped to change the hoses, when all this happened. We found that if you pump the accellerator, you only get two good sprays into the throat then no spray at all, the bowl is empty of fuel. I do have the accellerator pump in mind, it is newer but may be on the check list if improvement does not occur. with the OEM fuel system OOS, the system is very simple now. there is very little to test and find the source of the problem. We know the OEM fuel system starves the system of fuel, this happened since the tanks were dropped to change the hoses. but so much has been changed out to get the GMC running again that everything is now in question. Still trying to find the problem(s).

I'm going to asked 2 of the stupidest question on here probably.....

The motor DOES have a fuel pump eccentric still on the end of the cam? Guys have been known to remove them when swapping over to electric pumps and doing the timing chain set.

Is the pump being installed on the correct side of the eccentric if it does have it still? Yes it hard to do but it can be done, been there, was one of the craziest diags i've had to do.