losing coolant?

[setandreel]

Hey all,
need a little help, got a 65 with a newly rebuilt 250 and a new 3 row rad. from advance and a new heater core. Now here's the kicker, I have no visible leaks of anti-freeze any where, I have even put large sheets of card board under my truck. and nothing but I am losing a good bit of coolant, maybe a half gal. ever 75-100 miles, now I don't drive the truck a lot, sat. mornings to starbucks with the wife and church on sun. and just short runs to the store maybe 25 miles a week so it does take 3 or for weeks to lose the coolant. Any ideas? I am guessing head gasket but no coolant in the oil or vise-versa, and before I take off the head thought I check with the pros, so any help is greatful
Thanks
Mike