I agree with the Captain's observation about the early SUs having little to no problem. My early 71, for example. 20+ years driven daily, year round, never had VL, even once. Stock set up. No elec. f. pump, just the mechanical one on the engine. Pressure was slightly more than 3 psi as I recall. (BTW, we used to run a 327 in a dragster that had plenty of fuel at 3 psi.) Clear filter on the firewall was never full. It ran at 1/4 full or less. No vents in the hood. Seemingly everything that should have caused VL didn't.
So, we know the later round tops and flat tops were plumbed for hot water and that can contribute to VL. It seems that the addition of an elec. pump did little to nothing to eliminate VL, if that was it's purpose. Nor did the hood vents seem to help much. They did change the fuel rail from two lines to three later on. Could that play into this? I don't know the reason for that change. What was the third line for?
Drilling the restriction in the line is an interesting idea. It would be interesting to put a f.p. gauge on it before and after. You're not going to hurt anything until the pressure drops well below 3 psi. It doesn't take much pressure to fill the float bowls. There's no restriction until the needle and seat close. Usually running low on gas in the bowls is from lack of fuel volume not poor pressure.