should i swap motor,need more horsepower

i have a 68 plymouth valiant, with a 76 dodge 400ci big block with 72 heads, purple cam,fender well headers, 727trans shift kit,4.56gears.it does low to mid 12s quater mile. came into minor engine problem over summer, long story short got stuck in traffic with crapy oil in car and knocking started, still runs but pretty bad knocking, ive been told my engines maxed out for horsepower while still having reliability. should i fix engine and try to get more horse or swap for 440 and start fresh,iam only 22yrs old so i need to keep cost down,i do all my own work. any suggestions what i should do?