is a 3/8 fuel line big enough to handle a 600 hp engine? what are you using if not? have a snipper fuel pump in tank before you ask and no mechnical pump. have a holley regulator i was going to use a 3/8 line for return also. thanks for any help

I would look at the pump manufacturer's recommendations.
With a BG 240HR pump they wanted -8 delivery and -10 return.
The larger return surprised me , but it makes sense after kicking it around for a while.
3/8th will feed 600 h.p. , but I might like a little larger just because overkill and I are old friends.

my pump says 255 gph but the fittings on the sender is 1/4 npt out of there sender? going to restrict it there anyway?

With an in-tank pump you will be fine.

I've ran in tank pumps and 3/8 feed line at over 1000rwhp, fwiw.

The bigger the line the bigger the pump to push the fuel up front, inertia

The Magna fuel 275 pump has right at 18 lbs. of bypass pressure, their 300 pump ( I've owned and used both) looks identical to the 275 pump but it has 25 Lbs. of bypass pressure, if I'm remembering correctly now
Both want a #10 inlet with a #8 outlet, the bypass line from the outlet line on the pump going up to the return line to the tank is a #8 also