90's "muscle car" choices?

With the criteria you listed, I'd keep the Mustang. The 89+ models have mass air which makes mods easier, but the conversion for yours isn't particularly difficult or expensive.

That 80s GM stuff is pure junk, and the 50 extra cubes don't get you anything except worse mileage. LT1 has more power than the 80s stuff, but still all of the disadvantages in terms of repair complications.

I agree with streetdak about the LS1, but I haven't ever seen one worth buying for that amount of money.