Little help here please

[macdaddy70]

My 70 GMC with 307 and 2bbl Rochester carb with divorced choke. New tank, sending unit and new plugs, wires, points (set with dwell meter), coil, etc. Recently rebuilt carb because of this issue.


Truck idles fine, but when I cruise 20-30 mph it wants to run rough (jump and surge but never go dead) Get on the interstate and can cruise 60 mph with no issues. If I let off the gas and get back into it, there is about a second hesitation before combustion and a noticeable THUMP, then it runs fine.
Any thoughts on what to check next? When the carb was rebuilt. I did use the paper gasket and not the 1/4" thick gasket that came with the kit. Should I have?
For what it is worth, it is hard to crank after sitting for a few days...not sure if that is relevant.
Any help would be appreciated.