318/360 heads,with thumper cam

So for the past two weeks ive been reading,and reading and reading,but the more i do so,im left with more questions.What im gonna ask is how can i make the 318 i have now a better performer?As of two weeks ago its down to block,cam, and pistons,I removed the heads,and they appear to be 61,or 62 cc's(if i can remember correctly) the intake is a stock 77' 360 4 barrel,i have a 600 cfm edel,and also a holley 650 vac-secondaries,headers are fenderwell type that i will finish welding up as i get the heads back on,the reason i took the heads off is to get them milled,as i hear they are not too good on a stock pistons 318 motor,but i don't know how much they should be milled,i think i was lied to by the seller i bought the car from,he had said it was"fully rebuilt",if so how do i tell if the engine is bored over???the pistons are flattops with no valve reliefs.i want close to 300- 330 horsepower with using some of what i already have.