1966 Ford Fairlane Lost coolant

I have a 1966 289 I am losing coolant from the overflow. I have replaced the rad. cap added a overflow can that reclams the coolant and a new rad. I went with a lager 3 row rad. But after each step I still have the same issue. No water in oil no water from tail pipe no leaks under car. Just pushing water out the overflow and then out of can. im going to try a new trermastat to see if maybe its not opening all the way, any ideas? Thanks