Never did a B-body. I did a 5.3, and then a 6.2, in my '64 GTO. I modified the factory harness, and PCM, for the 5,3. It wasn't bad. Just time consuming. I learned A LOT! The went with Holley Terminator X Max for the 6.2. The Holey harness is much easier and cleaner. I still stripped a few thing out of it. But the PCM/ECU is plug and play. Self learning is nice. Plus runs on wide band for full throttle learning. If ya want to put in the time, the factory stuff works great. Especially on stock components. Check out:
https://lt1swap.com/wiringharness.htm
Next is A/C and a 5-speed stick. Couldn't have gotten away with A/C and 455 (no comments from the peanut gallery please). Probably not gonna drag race it much (or at all). So back to banging gears and factory console.
Good luck!