I have TFS TW heads on my 347 also. At first I thought they might be a little on the small side for my motor. After talking with my builder and reading alot of info on cross section of heads, it seems as though the TFS TW & AFR 185 are the most compared heads have an almost equal cross section. It seems that the 185's have a slight edge over the TW heads in OOTB conditon, but you can have a lot of head work done to the TW. It seems there are very few reconized people who can work an AFR head and have a huge increase. I also will be running boost later on, and I dont see why I cant make 600rwhp on pump gas with those heads and 15lbs of boost. Most of the info I got were from the cam grinder web sights ( flow tech induction - Ed Curtis & camshaft innovations - Jay Allen )