I have several non-AC 1970 underdash harnesses. In fact, there's another difference in the length of the heater box wires if the connector is at the passenger side of the harness. AC versions are longer by approx. 10 inches, so it can reach the left side of the heater box; non-AC heater box has connector on the RH side. That said, there are at least 3 variants of AC wiring for the 1970: RH passenger side, 3 pin flat half-way up the harness towards the firewall grommet, and a combination of the two. This creates real problems for me when someone simply wants to buy outright an AC harness...the only good solution I have is to provide the combination variant or the RH only variant, which ensures there's a connection to the resistor pack.
AC wiring became standard in Mustangs in 1972, whether AC was present or not.
I do not have a good inventory of Boss harnesses, but I believe they simply used standard Mustang wiring and added the rev limiter under the hood.