sputtering only under load CAN NOT Figure it out

[kserieskid]

hey guys ive got a sputtering problem on my 1981 chevy stepside with a 305 qjet carb and regular hei dist set up

the truck will only sputter under load, i can put it in park and rev the **** out of it but when you put it in drive and power break it it seems to run for a bit then fall on its face then come back and repeat. same when you drive it, during normal crusing speeds and mild/fast acceleration

what I have done so far
change the distributers cap rotor ignition module and the coil along with the plug wires, the plugs look fine had a mechanic at a local shop look at a plug and they seem normal.

plugged any vacuum ports open and moved the vacuum advance to a port right off the carb.

changed all the fuel filters and made im getting plenty of fuel

any suggestions will be a big help

distributer also has an advance curve kit it was there when I popped the cap off

thanks Kserieskid