At the track last night it would level off and loose power?

750 cfm carb? holley?

even with a 5/16 fuel line...should be enough fuel in the bowl(s) to carry you thru 1st gear......

my 73 "Duster with 750 dp is fed by a 5/16 line with holley high volumn mechanical pump....has run 106 mph in quarter...

timing or valve springs....if hydraulic cam....check preload on lifters...