These guys can usually give you a better price and product for radiators/heater cores and such..
Performance Radiator®
And also when installing the thermostat make sure the small hole/nipple deal is in the 12 o'clock position. This also helps bleed coolant.
If a headgasket does wind up being the issue I would recommend carrying the cylinder head to a machine shop because of the mileage. Have them resurface the head, check for loose valve guides, reseat all bowls and valves, replace valve stem seals and check spring tension on all valve springs.. This is alot of work but the car will be ready for another 250k mi when put back together.