fuel pressure dropping over time

diymirage said
its a 408 LA, the fuel lines run in the stock position
the regulator is mounted on the passenger fender and from there feeds into the carb, pretty standard

i would expect the fuel pressure to drop a little with under hood temps rising, but not this much
specially not with a return system and fuel constantly circulating

Well, you have the gauge that's telling you it's losing pressure for "some" reason. Heat is always the first reason I look for, especially if there are "times" when the fuel pressure is good. Have you pulled the dipstick and smelled the oil to see if maybe the fuel pump diaphragm has ruptured?