engine power

Was this a new/rebuilt transmission and torque converter or was it used? I have seen many problems with used or 'bone yard ' transmissions. Is there a Check Engine Light ever on? This could mean that a wire or ground was not installed correctly.

their are 2 harness connectors that are identical looking and if you get them mixed up you will only have 3 of the six cylinders firing. Check the injectors 2,4,and 6 I bet they're not firing! You will have to trace the harness for them and swop the 2 connectors and things will be great!