background is rebuilt 1500 miles ago (no new sensors ), running t34, 803's, msd chip

builds up fine, but when i try and rev it more, say past 4k, starts to miss a little and hesitate. will till rev up, but just doesn't feel smooth. have turned boost down from 20 to 13, to see if it was blowing the spark out, but no different.
popped the plugs out, 1,2 and 3 were quite black, but with a light brown tinge on the ground terminal, 4 was mainly black. couldn't adjust gaps due no knowing the right distance, but all 4 were the same.

its running ngk iridium plugs, magnecor leads of unknown age, fairly new dzzy cap and rotor and a 500 mile old grp a coil

any ideas where to look next? want to sort it before i go for a dyno setup

cheers
Dave