poor acceleration up hills

My sister has a b2200 2 whell drive truck and it won't accelerate up hills it won't rev high enough to shift down to a lower gear when going up hills like most cars and trucks do. it doesn't go beyond 2600 rpm's when gas pedal is to the floor in drive or overdrive. Can't seem to figure out what it could be? Have rebuilt the carb, changed spark plugs and wires adjusted the timing adjusted the idle screws checked most of the vaccum hoses for right pressure (21 psi) and everything is hooked up . If anyone could head me to a different path that would be great thank you.


Miles

You mentioned everything but changin the fuel filter, if haven't changed it yet now s agood time.