Make it work

hello; I have a 1997 GMC Z71 tuck that i have gutted and am installing a 2002 2500 HD complete package, motor and trans and wiring inside and out and fuel tank complete. i got it running, got computer flashed and it idle like a kitten but pulls like a fly. My tach is reading about 1100, 1200 at idle and will peg out when shifting gears. My snap on versus is not reading on cylinder 7, 8 saying N/A and has sever codes in it about o2 sensor lean. I put new fuel pump in also. ( is it possible that my computer has been flashed and running with V6 program and that is why my tach is high and not recognizing two cylinder)?
Thanks

76larkman

Look into Lt1Swap.com for wiring info. If nothing else email him on your situation. Why didn't you make the engine harness standalone?