Chevrolet Repair: Engine swap

Question
What part of the wiring harness stays and what part do I take out when doing a engine swap from a 1997 Chevy silverado pickup 305 engine to a 2003 5.3 engine. I have the new harness but don't know how to tap into the inside cab dash functions. I have the new computer and accelerator pedal/module all plugged in as well as the new harness but old harness runs in threw dash where new one only has a couple wires to run to the inside of cab. Do I unplug the whole old harness out from under dash?

Answer
These questions will be very difficult to get the answers to in my opinion. You might get the helpful info you are seeking from an in house engineer at GM in Detroit. I would have gone with the same year engine with harness.I have spent hundreds of hours literally studying and troubleshooting this Cadillac I owned.Poured over all available schematics for the electrical on that thing. I can tell you this fairly confidently : an overwhelming majority of technicians wouldn't be capable of giving  a right answer to these questions you're asking. It will be a very difficult task to get every thing to work without a hitch. Get something wrong and you can be buried by all the possibilities of causes. For example A turn signal wire miss pinned so as to not match up with the correct corresponding socket can disable your truck or even cause it to malfunction in ways  even the engineers cant comprehensively explain initially So .sorry and good luck.