"PLEASE" Help my Chevy driving brother.!!

[Dark_5.0]

My dad and brother just installed a set of sportsman 2 heads on my brothers 81' Camaro. He has the following motor and mods.

406 small block, performer RPM intake, Holley 750 carb, comp 491 lift cam, 1.6 roller rockers, Headers, Holley fuel pump,
World products sportsman 2 heads, Turbo 400
with 3rd degree shift kit, 2500 stall, 3.73
gears.

After they installed the heads, the motor rumbled louder than ever and had real quick throttle response, so they couldnt wait to get it out in the street. They stalled it up
gunned it and tore the pavement off the road
but then hen the car got over 3000 RPM's the cars power was gone. When he would shift through the gears the car wouldnt pull for
sh!t. I dont think that the problem is in the motor I think that something is wrong with his stall converter. The car ran 14.30's before the heads and now it would be lucky to run a 15 the power is not getting to the wheels.

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88 mustang gt convertible bored to 306 BBk equal length headers mac cold air fender kit cowl hood nitto drag radials ford racing clutch flow masters h-pipe 160* thermostat 16 degrees of timing electric fan off 94' t-bird jacobs electronics cap and wires. Edelbrock 1 1/2 inch drop springs.
best 60 ft 2.14, best 1/8th 9.65, best 1/4 15.2 @89mph, Stock gears and 3000+ Altitude