How to get 400 hp out of 1971 340 .30 over

I have 1971dodge demon 340 that is .30 over flat top kyb pistons ,comp cams 268/280 ,750 double pumper, edelbrock rpm air gap port matched and headers 3.73 gears and with automatic. My engine builder said it might make 400hp but he said it's hard to get that out of 340 unless you stroke it. Why is this so hard.