approx when do the stock fuel lines become a restriction in feeding a motor?

 

It depends on your pump, but generally speaking I think it's around 450hp.

 

A good mechanical pump on the engine will feed that through a 3/8" line. A good pusher electric back by the tank will easily feed 500 HP through a 3/8" line.

I don't have any problem with 3/8" line & the 396 in my '57, which is putting out an easy 400 HP.

 

i found that my fuel pressure would drop around higher RPMS through my factory lines with an holley blue pump when my motor was about 450 HP last year(nitrous included). This year I switched to two holley blues and a sumped gas tank with 1/2in line for my future motor plans this winter. Hope this helps.