95caprice254

Ok, this may be a bit long but here we go. I have a 95 caprice, it came with the 4.3 but I recently put a LT1 from a roadmaster in it.......I changed, water pump, air compressor, coil, distributer, spark plugs, wires and fuel pump and I changed the computer today......i also checked the firing order at least 10 times......the car starts great, but once in gear it does not accelerate well, and it sputters.......I have no idea what could be wrong can somebody please point me in the right direction.......