Just a thought, and my 2-cents...
Oftentimes when a problem pops up, it is very likely that the most recent change to hardware or other maintenance makes the problem appear in the first place, so going back to what was last done may be the issue.
You mentioned you are using an electric fuel pump- is this recent? Do you have a fuel pressure gauge (gauge AND regulator is better) to actually see what the fuel pressure in the line actually is? Fuel delivery pressure is important, and not all electric fuel pumps are created equally- some are quite poor in their basic job of a steady pressure and fuel delivery.
Has your mechanical fuel pump been removed from the engine, or is it still in place? If still in place, is your fuel line still connected inline or has it been bypassed?
Do you currently have a fuel filter on the pickup tube in the tank? You mention some "stuff" in the carb pre-filter- it's coming from somewhere upstream, most likely the tank.
If the innards of your tank is at issue (most 50 year old tanks should probably be replaced), then I would suggest this as a thought to pursue. Check out
http://www.tanksinc.com/index.cfm/pa...rod/prd404.htm
Anyone out there who has "dressed up" their fuel delivery lines with the fancy (and $10.00 a foot expensive) should think really hard about removing it and hard-plumb it. It is nothing more than cheap rubber hose made in China that is covered in the braided stainless, and is not even compatible with gasoline for long periods, and I am a testament to this fact... I was backing my 55 Chevy out of the garage recently, and smelled raw gas- found a huge puddle under the engine, and it was soaked. Fancy SS covered fuel line has split. Can you say FIRE? Well, there wasn't one, but there easily could have been.
Not to worry-
NAPA has some hard line that is very malleable, because it contains some copper, it is easily bent, lasts forever, and is nice to look at with a very subdued gold look to it.
RUBBER FUEL LINE is just fine to use, just not the junk that is masked with the braided stainless mesh, at least the stuff that I had under my hood at the time. It just needs to be stamped as "fuel line", because that is what it is made for.