Realized this is way too long for most to read, so here's the abridged version:

-Intermittent problem w/ van staying running
-Starts first crank every time, but will stall out after 20 seconds up to 20 minutes
-Ran perfectly prior to installation of a new electric fuel pump and elimination of manual fuel pump (plated off) - not sure if there's a correlation here or not
-Just tuned it w/ new plugs, wires, cap, rotor, pcv
-Have blown out the fuel lines front to back, attempted to power off an external gas tank, even removed exhaust thinking it may be plugged - no luck

Help!

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Long versionGreetings all. New to the forum w/ a 1977 Ford E250 hi top camper van w/ a 460 an 79,000 original miles. I picked it up from an elderly gentleman who used it as his fishing rig. It's been used sparingly for the last decade - 3000 miles or so. His health was in decline, thus the reason for sale. She's in great shape in and out. Here's the problem:

Drove like a champ on my 30 mile drive home. Started right up and had no issues. The next week I tried to start it up and had a hard time and it wouldn't stay running. In retrospect, I believe I wasn't operating the manual choke correctly (gas pedal has to be depressed to achieve full choke - I was only half-choking it it turns out). Decided to bring it to the shop to get the fluids changed - oil, coolant, brake fluid. Also had them look it over. They said it had a leak at the manual fuel pump and fuel line was a bit rotted in that area. Also said the electric fuel pump that had been installed was a cheapy and recommended replacement and plating off manual fuel pump as they couldn't find a replacement (said it was on national backorder). Ok, fine. Go ahead.

Picked it up, drove home (4 miles) w/ no issues. Gave her a tune-up (plugs, wires, cap, rotor, pcv). Took her out for a spin and it ran great. Decided to see how she'd do under heavy load and gave her a good womp on the gas. Not long after it began to sputter then stalled. Ever since then, it will start up like a champ, first crank, run for a bit, then sputter out. Sometimes it takes several minutes (10-20) then it'll die. Sometimes it will start and stall out w/in 20 seconds.

Brought it back to the shop. Long story short, they reached my financial cut-off point at which time they still didn't solve the issue. Blew out the lines, tested the fuel pump, even disconnected the exhaust thinking it may be plugged. Still nothin. Next steps were to drop the tank to see if a vent was blocked (I kinda thought this would be a 1st step, but what do I know) then rebuilding carb (quoted 500-800). At that point I pulled the plug.

W/ all that said.... any ideas? I'm at a complete loss. Shoulda abided by the 'if it ain't broke, don't fix it!' rule from the start I think! Thanks in advance for any help/advise.