...I see that on alot of conversions, where the stock gas tank is used, that some people use a low pressure electric pump that feeds a small tank with either a high pressure inside or just outside it, inline to the engine.
True. it is popular because you don't have to do any mods to the tank itself, or mess with fuel level senders, etc. The whole requirement on EFI is NO AIR BUBBLES. The surge tank and other accumulator systems provide that with any fuel tank type and in high-angle (off-road) or high-G maneuvering. Contrary to popular opinion, they are not cheesy get-by devices. They are commonly used in serious race cars for fuel feed reliability.
I was wondering if you could use a mechanical pump to feed the high pressure pump.
Absolutely. BTDT. Doing another right now. Remember, the surge tank is fed volume fuel from the tank but with no pressure. Stock fuel pumps (like all pumps) increase their pumping capacity substantially when there is no pressure to make. You'll have plenty of volume.
My thoughts were to make a stainless tank that would be about 4" diameter and 8 to 10" long, similar to a cool can with the high pressure pump inside and mount it at the front beside the rad.
You can buy what you describe for race cars at only $500+.
Your design is fine, but a bit large. Half that size will feed 450hp for nearly a minute with no LP fuel replacement. I build mine at 1 quart maximum, as any more is a waste, and is good enough for a serious drag or road race car. Easier to tuck away, too. I like to put them at the front of the fender well where the Cougars had their big vacuum cannisters mounted. That puts it out of the engine compartment heat as well.
That way I would have one less pump to worry about and the return line would not have to go to the back.
One less pump is right. You already have a reliable LP pump mounted to the engine - why not use it? I do. However, you must have a return line to the tank as the fuel will get really warm really fast. A typical high-performance HP pump makes more heat than a 100 watt light bulb and the fuel cools it. As already mentioned that will cause issues without a return. I have heard some that have used a small radiator with a returnless system, but I don't know how well that really worked. A return line also avoids any pressure in the surge tank.
The only major drawback that I can see is that there would be about 1/2 gallon of gas at the front, which in a crash would be a hazard. What do ya think?
Well, consider in a carb you have fuel, and the lines will drain if they or the container are opened. Besides, the FAA approves surge tanks in aircraft engine compartments and even under the dash inside the cockpit
as acceptable for safety. I have no concern and I've never heard of an issue with them (and many factory cars have them), especially with an electrical impact kill switch (standard Ford item), but your call on that.
Oh, look! A surge tank ready to be fabbed! LOL There are many items in many materials that are good for making surge and accumulator tanks. I prefer an external pump for fabbing simplicity. The pump is off Ford E and F series trucks/vans, comes stock in a convenient bracket and the thick sound insulator just below it. You can hardly hear it if you're trying, and they support 450hp stock. Next to that I put the important impact switch, available at your 'yard cheap or auto parts store for under $40 new. Finally the remote fuel filter shown (from many commercial vehicles) is modified into an air-separating surge tank, is cheap and easy to do. Lots of options!