How much horse power will the factory fuel line support.I have a built 400 small block, and I think its running out of gas at the eight mile. Im running a holley mech fuel pump.

The line will support about 500 HP.

An engine-mounted mechanical will suffer trying to suck through the line. A pusher back by or in the tank will help a lot.

Assuming you still have the stock Qjet...If it feels like you are running out of gas it be that your carb bowls are running being sucked dry before they can be refilled. Qjets are known for that issue. Also on Qjets, not large enough secondary rods will cause fuel starvatoin at higher RPMs when you get on it. I solved it by installing larger secondary metering rods. This is assuming your 84 has the stock Qjet still on it.