1970 340 heads on a 73 318

Hi
I have a set of 70 340 heads that I was going to put on my 73 318.
But now I am confused with all the different opinions. I have read that I will loose compression with the 340 heads because of the larger combustion.
I have been told to put thin head gskts on and it will be ok. I also noticed the combustion cc is smaller on the pre 72 head. Will this setup work. I really don't want to deck the heads. I just go to car shows and want more power than the 318 gives me now. I am currantly running edelbrock performer intake and edlebrock small 4 Bbl carb. Dual exhaust (no headers and stock cam)
help! :munky2: