Bigeats6.0

I just recently bought a 05 Silverado with a 6.0 and from what the guy said a 4l70e out of a TBSS. Well the problem I am having is, cold or hot, under 25 mph if I go WOT it acts like its jumped into neutral. Now it is supposed to have a 3600 TCI stall in it, it has a stall but not sure if it's a 3600. It is supposed to be tuned with 42# injectors. I just want to get this trans issue resolved. Anything would be helpful, maybe someone else has had this problem and could shine some light on the subject for me. Thanks in advance