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

At 46-4800rpm? 1st gear she would go to 5300, 2nd gear level at 4700 and 3rd gear would not go past 53-5500 rpm s. Cool 60 degree night.
Timing? Now at 15 initial all in at 2500.
Thanks in advance for your thoughts.

1971 scamp
My build
360 la internally balanced with balanced flex and harmonic.
Stock crank, 340 hp rods
Comp cams 280H
Kb 107's NOT zero decked .012 in the hole
Prw stainless 1.6 roller rockers
Indy heads set up by IMM/ Brian 70cc chambers .29 head gasket
Edlebrock air gap intake and 750 cfm carb
Miloden pan and mp windage tray
Dougs headers , summit 2.5" exhaust
PTC 25-2800 stall torque converter and transmission full manual
8.75 rear with 355 gears posi , Mancini adjustable pinion snubber
Mp Ss springs and shocks