'71 318 compression...

Hi, I'm a newer member, and first want to say this is the greatest site!! I've learned alot...just bought my sixth mopar (most "back in the day") It's a '71 duster from Arizona. Original 318 2 bbl., does have duals., fresh brewer's 4-speed in Nov., 8.25, 2.76 gear.(plan 3.23 sure-grip later). I'm getting ready to order some bolt-on's and thanks to this site, know not to go overboard... 600 carb., 340 cam/lifters, dual plane, d.r. chain, perf. curve dist. My question... my heads are 8.6:1, 60 c.c. chambers. I found some thru aeroheadracing, $400/pr. that have bigger stainless valves but have 67-70 c.c. chambers. They told me it would not effect compression, expect 15-20 hp. This is a budget street build...keep what I have? thanks for input...