head issues

my 89 s-trim f cam has been pushing coolant out the resv. on an ongoing basis. it never gets anywhere near overheating until the coolant level has dropped so low it can no longer be pumped through the system. there;s no sign of coolant in the exhaust or water in the oil, but the coolant does get kind of foamy. I haven't done a compression test but I'm thinking i have a head gasket leaking into the water jacket but only under combustion presures. any way i came up with the idea removing the rad cap, taking a home CO monter in a plastic bag, sealing the bag over the rad opening and seeing if i get any reading with the car running. surprise surprise, i did. I'm thinking this this confirms my suspicions. Anyone out there agree or disagree? i can't see how esle you'd get CO in the coolant system.
So with this in mind I figure it's time to upgrade the heads while I'm changing the gaskets anyway. I was thinking of using tfs 185 cnc units but they aren't avail machinrd for the loc wire gaskets like the 170's which i really think i should use. does anyone have any experience with this? are the 185's really worth the extra six or seven hundred they're gonna cost? if so any reccomandations on a good and reasonably priced machine shop th the northern nj area to cut the heads for the gaskets?

any input would be greatly appreciated
thanks guys

the rad cap was one of the first things I looked at appears to be sealing fine but replaced it with a new 16lb just for good measure. I said its been doing this for a while but i guess its been about two weeks and i've really been babying it. i think i gonna just break down and go for the heads