What should I do to my '91 5.0?

don't bother with the MAC cold air kit. factory air box IS a cold air system so you won't see any benefit from it aside from looks. 3.27's suck unless you have a big power car. go with either 3.73's or 4.10's. you should definitely get the gears, shifter and full exhaust though. great improvement over stock. IMO, go with either a pro-5.0 or MGW shifter. my friend has the hurst billitt plus shifter in his coupe and i don't like it was much as my pro-5.0 at all. if you're looking into getting a set of heads, don't just do heads, get a matching cam and intake. the trickflow top end kit is a really nice kit. if money is there, i'd look into that.