With no stat & back out in the same driving conditions the overheating problem dissappeared? That'd almost sound like the stat was upside down or something in the stat housing was keeping the stat from completely opening when it was in there or just a bad stat & possibly the second stat could be bad also. Assuming the timing is in the ballpark & I'm thinking that it is I would have a rad shop pump it up to 15 lbs when hot & see if it holds pressure (head gasket/cracked block or head(s) & if good there then as said the radiator. I'm assuming it runs hot around town and out on the hiway?. Holler how it goes. EDIT FastMark may be on to it (but I would say head gasket cooling holes) especially if it did not overheat before it was rebuilt & now after the rebuild it does overheat & the same rad was used. PS how hot is it getting?