350 Chevy running rough, lots of time and effort spent

[thewrai6th]

I have a 69 GMC with a 350 (85 Blazer block?)/3898993 heads/Holley 600 carb, fuel pump and intake.

It has been running great for years.

The truck seems to start and idle fine. As soon as I take it down the street, it starts running really rough. Idle drops and it almost dies. If I shut it off for a few minutes, it's fine again until I drive it down the street. No choke, manual or electric, never need it before. Choke is wide open.

This seems to have happened when I ran the tank low on gas. So I've replaced the fuel filter and found alot of rust and crud in the old one. I took out the FSU and blew it out with compressed air.

I have new plugs r43ts ACDelco, .035 gap. New wires, cap and rotor. Fresh coolant and oil. Timing is set at 8 BTDC. I can't find any exhaust or intake leaks. I was thinking maybe the wires were overheating, but I don't think so. I checked each wire with a timing light and they are all sparking.

I'm running out of ideas and I'm about to do a leakdown test. Seems like a fuel problem to me though. Any suggestions?