Might it be the pressure relief valve on the return side of the IP? Maybe not enough system pressure at cranking revs but enough once the engine catches. I read about pinching the return line between the big filter and hard line on the fender (cigar hose on a 617) to see if the engine is easier to start as a test of the check valve.

Can you do a basic test of the lift pump? Compare output at cranking rpm between drawing from the tank and drawing from a fuel supply higher than the lift pump.

Have you ruled out a clogged tank vent?

Sixto
87 300D