fuel delivery issue

Not clear--can you determine if pump pressure is dropping when the problem shows up? The pump should have enough reserve that it can maintain pretty much regulator set point pressure. If not, something's restricted, vapor lock problem, or the pump is falling off. Could be something like a worn or loose cam or a pump arm tryin' to fail. It happened to me on a 440 (pump arm)

Now you say "the tank is not vented" and I'm not intimately familiar with 'Cudas but I thought ALL pre 70 A bodies were vented via a vent welded into the fuel filler tube. (70, I believe, was a split year for the addition of the carbon can system and "pressure vacuum" caps) Since your body went "E" in 70, I don't see how that could be an issue, unless the vent was removed, or otherwise somehow plugged (bugs, gum, rust, etc)