Engine problems .... need help

[71sub]

I have a 72 Cheyenne LWB -- 350 / 700R4 w/ 3.08 gears.

I've changed the spark plugs, wires (loomed), cap & rotor, carburator, electronic control module ( HEI distributor ) and checked timing.

When driving and try to accelerate I loose power -- I get a pssssst noise comming from the right OR left side of the engine.

Also, I can't take any grades on the freeway above 40MPH w/o giving it too much gas -- otherwise it seems to sputter and pssst.

I'm running out of options --- it's less pronounced when I put the tranny in 3rd and NOT let it cruise in O.D.

Any help would be greatly appreciated.

Thanks

Ev