overheating problem with 350 small block can someone help please??

Dear Reader, I own a 1978 Oldsmobile Cutlass Supreme 2dr RWD. When i bought it the stock engine was already swapped. I did a tune up on it (just plugs wires and coil) and in the midst of trying to get the right parts for the tune up i was able to identify the motor that is in there as a Chevy 350 small block from like a 1990-1993 chevy pickup (it has fuel injectives). I also replaced the radiator with a brand new radiator. I had been driving the car back and forth to work every other day approximately 30-35 miles one way. The engine sounded fine and ran fine, matter of fact i was outrunning most of the new stuff on the freeway. One day i overheated about 5 mins from home so i pulled over let it cool off and took it home. Since then i replace the thermostat and went about driving it for about 4 more days until it ran hot again. i was running with out a overflow tank so the hose that i had blocking off the overflow passage had burst and i lost all my coolant out the passage where the overflow tank was suppposed to go. i bought an overflow tank, replaced the radiator cap, and refilled the radiator again, and replaced with yet another thermostat. it ran hot again, but it takes it like a good 5 or ten miles of strenuous driving b4 it runs hot but when it does run hot all the coolant bubles into the overflow tank then out the overflow tank along with steam. I totally removed the thermostat while it was hot one day and held it up to cool air with tounges and watched it close so i knew it wasnt the thermostat. i ran it without the thermostat at all for 5 to 10 miles and it overheated again. i checked the oil, there is no sludge or signes of moisture, the coolant looks fine but i loose almost all of it out the overflow tank everytime i run hot, and there is no water coming from my dual pipes. This is soooo frustrating because i have put so much time and money into the interior because i thought the motor was straight. I love this car, what could it be, how do i fix this???? im no novice mechanic but i will get under that hood and go to work, somebody help please. Please tell me its not a head gasket!!!!!!!! whoever put that engine in did such a good job converting it and getting her running right, can you help, keep in mind she still runs strong as a bull, she just overheats too quick.