The return line is off the pump. My 74 didn’t have the return installed. Never had a problem for many years like 25 of them. Then it began, studder stammer, hard start hot, bucking. The hotter out the worse. I tried the non E gas, little better at best, so I’m not sold on the non E thing. Tried high volume EDL street pump no help ( no return line.) Changed Distributor, rerouted fuel line away from block, left the gas cap off, No Help. Last thing was to install a Wix 33041 fuel filter up at the carb inlet as a test. Has the 1/4 inch return on the filter with 3/8 inlet and outlet. My K5 did not have the return setup so I used the 5/16 vapor line that normally goes to evap can for the return. Gas cap is vented to prevent the return fuel from pressurizing the tank. After I did the fuel filter trick I did not experience the same problems, ran like it did years ago. It was for a short time due to a blown rear pinion. Been in the garage for a year for some work but it sure seemed like a fix. I am confident it’s a vapor lock problem due to modern fuel. I can’t speak for your issues but it’s worth a shot. Even though you have a return at the pump, you could try moving the return to the inlet of the carb just to test. Just a note I have installed a fuel psi gage at the inlet to the carb and every time the problems happen the fuel psi drops off to zero. After the filter/ return test the fuel psi never drops to zero. IDK but until I get back on the road that’s all I’ve got. Bye the way the return in the filter is a .060 hole. The pump still maintains 6 psi at idle. Best guess is vapor lock from the pump to the carb. I will also note that if I let the engine idle in the drive way on a hot day the clear fuel filter I installed would go almost dry and you could see the fuel splashing through, although the engine would never quit. If I put a water hose on the pump the fuel filter would fill back up to half way so it’s definitly heat related. That’s my experience, never had up hill issues and am not responsible for your results. Just IMHO.