If it does have a tank check to be sure that radiator cap and all the connections to the radiator are air tight. If any of them leak air then as the engine cools the coolant in the tank will not be drawn back into the radiator.
Mismatching the pulley sizes can do a number on how the cooling system works. Take a look at my web site in particular
http://www.canadianmilitarypattern.c...ifications.htm which shows what can happen with engine change mixing of parts.
The thermostat, can also cause the problem you mention, it needs to have a little by pass or the back of the block gets hot before the thermostat starts to open.
Take a few pictures of how the cooling system is set up. I sure the collective heads full of knowledge will help spot the problems. Then you make a list of the solutions and put them in order of easiest to hardest.
One other quick check is how thick is the core of the radiator. I've seen one with a Chevy car radiator instead of truck.
Cheers Phil