Engine hits wall at 5k only in 3rd gear

Sounds like you're emptying the fuel bowls faster than the pump can fill them. Get a fuel pressure guage and see what the pressure is when this happens. Shouldn't drop below 6 - 7. Also since you have an electric fuel pump try this:
disconnect the fuel line AFTER the regulator or at the carb. Get a one gallon jug and stuff the fuel line in it. Turn on the pump and see how long it takes to fill the jug. Should be somewhere near 30 seconds or less. You might be at the ragged edge with that 5/16 line. Or the pump might be bad.
Then again it might be something else entirely. Good luck, post what you find.