? abt my 93 gmc c1500 6 cyl?

i have a gmc c1500 with a 4.3 v6 in it.the truck has 257,,000 miles on it.i have a used 350 chevy im going tobuild up and put in it for the street/strip when im done school and find a job as a welder.now my ? is abt the ani freeze.i keep having to poor sum i every few weeks.i replaced the heater core a few months ago as it was leaking out of the heater core hose when it got warm.can any one help me out at what is happening to the anti freeze?i cant find any leaks on top of the motor unless the heads are leaking alil i know its not going to the oil.im just trying to find out what the problem is atleast get it to stop using the anti freeze until i get my 350 together.