why would they be disconnected? the stock O2's are beefore the cats on the OEM pipe and there is none after, just swap over your stock O2's to the new pipe
You would hook everything up just like it was factory...except you will prob. want to use some high temp hose to run from the air pump hose to the tube on the O/R H.
The reason 96+ use MIL's is because they have 4 o2's. 2 before the cats and 2 after the cats. These "rear" o2's measure how much the cats cleaned the exh. vs. what was going into the cats. The computer checks the readings on what the front ones read...then what the rear ones read.
Since I do not have a 96+ and never actually installed MIL's on any of my buddies cars that are 96+ I do not know if the MIL's replace the front or rear o2's.
Either way if you have a 95 or older v8 mustang you do not need MIL's and you will keep your 2 o2 sensors hooked up.
BTW- it is common for "dumb" parts store people to insist we 94-95'rs have 4 o2's on the v8 models because thier computer says we do...that is NOT the case we only have 2 o2's. I think the confusion comes from the 94-95 v6 being OBDII and they probably, I assume, have 4 o2's (I do not know for sure if they do have 4 as I have never actually looked/never had a buddy with one).