1968 GMC 250 Inline 6 4.1L Chugging

[68gmcdude]

I've reached the end of my patience... ��. My 250 inline 6 starts and idles fine. It will run down the road like a champ for a mile or two then start chugging and backfiring real bad. When I let off the gas it idles fine then it starts chugging and backfiring again a little ways down the road. Here is everything that has been done...

New gas tank and sending unit 3 years ago
Carburetor professionally rebuilt fall of 2020
Distributor professionally rebuilt fall of 2020
New cap, rotor and points
New AC Delco plugs and wires last summer
New coil this spring
New fuel pump (last summer and again yesterday)
New fuel lines
New carburetor fuel filter (last summer and yesterday). Old ones looked good.
New vacuum advance line for distributor to carb

I'm going to pull the sending unit out of the gas tank. It's been suggested that maybe the sock filter has collapsed (?).

Any other suggestions or possible issues are greatly appreciated! Thank you!