Your diameter concern is only in the reduced pressure section from the regulator to the carb, and 3/8" will work there. From the pump to the regulator is higher pressure, so 3/8" is plenty for that run. As a more extreme example, this is why an EFI 5/16" line can feed double your flow at 6x the pressure your carb uses. More pressure = more flow in the same size line. Caution should be taken that many pumps (not all, such as the Holley Blue) are rated at 0psi (zero) pressure. Pump flow capacity will reduce as pressure increases.
Geek food follows. Skip this if you only want the answer, without the why:
If I want to prove this, in my car with my parts, fittings, filters, advertised numbers and all, I test the regulator return flow. The return flow is the unused excess fuel that is available for use at the regulator's set pressure. Yes, you can monitor your fuel pressure at max demand, but that's difficult for most. If you can, great, and more data means more confidence that you have no issues or can spot them before something ugly happens. This test finds issues before you leave the shop.
Don't do this - this is how I do it and you don't want to do something stupid like me. I run the return to a suitable container using fire precautions, and time the return flow for a specific period. Let's say I get 32 ounces (1 quart) in 15 seconds. In one minute, that's a gallon. In one hour, that's 60 gallons, or 60 gph. Is that enough? Let's see:
HP x BSFC ÷ Pounds per Gallon x restrictive flow compensation = Required flow in Gallons Per Hour
For 525hp, at a typical BSFC of 0.5 pounds per hour, would need:
525 * 0.5 / 6.2 * 1.2 = 51 gph
OK, so let's break that down. BSFC is brake specific fuel consumption in pounds, or how much fuel is needed to make 1hp for 1 hour. Look-up typical values for your engine or from your dyno sheets, but they can range from roughly 0.4 to 0.6 pounds per hour, and most of our engines are around 0.5 at peak power on gasoline. So, 525 x 0.5 = 262.5 pph required.
Another caution, if you are running a low-energy fuel like E85 that needs a lot more volume for the same power, you need to change the BSFC and weight for that fuel. You may also require larger lines to feed that volume, so do your homework.
A gallon of non-ethanol gasoline is about 6.2 pounds. So 262.5 pounds divided by 6.2 per gallon is 42.3 gph. That's actually what the engine needs for 525 actual hp. So, what's the 1.2? In order to have a real-world buffer to compensate for low-voltage, dirty fuel filters, long-term fuel pump wear, etc, we add 20% extra to cover for stuff like that. Insurance. Some simpler calculations don't have this specified buffer, and instead use lower fuel weight or other skewed number to get the same effect with a simpler formula.
42.3 x 1.2 = 51 gph. We have 60 gph return flow that exceeds our 51gph requirement, so we have the flow to feed 525 hp, at required pressure, with buffer, and still plenty to spare. 😁 We also easily have enough to run common pump gas E10 and similar. I hope that helps in both sizing your fuel pump and lines, and how you can do more than guess and buy oversized stuff to be sure, and still wonder if it's enough.
