A few thoughts:
At a pump pressure of 5.75 psi AT THE TANK, the line pressure drop should be sufficient to drop the pressure seen at the carbs to less than 5 psi. Any major US brand carburetor in proper working condition will stand this pressure. The pump doesn't care if you are feeding 1 carb or 30, provided the total volume required is less than the 72 gph pump rating.
For fuel lines, since you have 3/8 to the carbs, and 5/16 into the carbs; why not simply take a page from Pontiac - the 1959-93 Pontiac used a 3/8 feed into a four-way (available from the Pontiac vendors) with 5/16 lines going to the 3 carbs.
Remember that by law (and a good one - MOO) permanent wiring on an electric fuel pump must come from an oil pressure switch (safety feature). You can still install a manual override push button switch to prime the carburetors.
As far as pressure regulators are concerned: - the testing we have done indicates the inexpensive dial type regulators will regulate to the dialed pressure ONLY AT RATED FLOW! At minimum flow (idle) the pressure will be equal to or very close to the output pressure of the pump. We do not sell or recommend the use of this type regulator (except for paper weights).
If in doubt of the pressure, a fuel pressure gauge may be installed at the line junction to the carburetors.
There is a relationship of carburetor fuel valve orifice size to the amount of line pressure which the float circuit can withstand without leaking. Unless this is a race vehicle, the original fuel valve seat specification will be adequate. You will not need to install a "high performance" fuel valve in the carburetors. In fact, adding the larger seat in the carburetor often creates flooding issues.
Sounds like a fun vehicle!
Jon.
