Steve, I completely agree with your thought process. Lose the NOS and the Haltech ECU. The car will be much more drivable with the Holley ECU. Adding a MAP sensor is pretty easy. You need a MAP sensor plenum, MAP sensor, fittings and hoses. Rubber hoses are fine. IPSCO used to make a nice MAP sensor plenum. It’s one of those items that was never on their web site. The Holley ECU also needs both a Cam Sync and Crank signal, either from a dual sync distributor or a separate cam synchronizer and crank trigger (which would require distributorless ignition). The latter is better but more work and $. It may also require different TPS and CLT sensors. Maybe a different O2 sensor too. I assume the car gets vacuum for the brake booster from an electric vacuum pump.
Maybe Mark Bailey needed Drive-by-Wire!
Good luck !