Od 700r4 Trans

My son has a 1989 K2500?? with a 700R4 trans. He bought the trans from a junk yard and it seems to work fine, but he feels it's not going into OD. I was wondering if the torque converter has anything to do with it, He did not use the one that came with it, he used one off a turbo 350 I think it was. It was physically smaller than the origanal one, but like I said it seems to work but for that, were also told that the cable has to be adjusted just right, any help with that will be of great help. Thanks..:banghead: