1979 Pontiac Firebird 4l60

Ok I have a 1979 trans am that had a newer engine and trans in, the engine and trans is out of a 1987 chevy camaro. The question that I have is that my trans wont shift right. In order for it to shift I have to rev it out and at the rpm peek it shifts I will do that from first to second but I dont have third and fourth. The fluid does not look burnt and it dont smell burnt. Other than just rebuilding it cause I know thats what I am going to have to do what do you think would cause it to do this?