[68gmsee]

Agee with above suggestions. And definitley could be several things from simple filter replacement to bad cam.

If you didn't drain the old fuel and cleaned the tank you may be having problems with rust and varnish build up in the tank. Since you've been running it like this it's possible it may just be messed up filters or the carb fuel bowl. Start with the easy ones like filters.

Here's a quick list of what I'm doing on my 69 truck that's been sitting for 5 years.

Check all rubber hoses for cracks and leaks

Replace plugs, points, dis cap and rotor and plug wires

Adj timing after engine replacement.

Insure the vacuum advance is working properly and not leaking (you can use mouth suction to check it)

Check fuel pump output by removing dist to coil wire, remove line at carb and place in a plastic jug, crank the engine. Should get app 1 pint of fuel in 30 seconds or less)

Remove old gas, clean tank or replace it