A c4 limits you off the highway if you also have any decent gears. 1:1 ratio winds up real fast without overdrive.
Build one yourself. It's _easy_. In fact, the hardest part of all will simply be carrying the 200+ pound transmission to the shop. $800-900 in parts will build you a fresh rebuilt, hardened shaft-ed, a+ servo'ed, kevlar banded, transgo shift kitted, 3K stalled AOD. The rebuild is basically taking snaprings out with a screwdriver and unstacking things. Then you slide some clutches in and stack it back up. It's really that easy. Damn thing just weighs a ton. People hate AOD's so you can get a nice core for _pennies_. I got mine for $100 from an 1986 grand marquis v6. Just be careful cause some of the v6 ones have a b servo and 1 less direct drive drum and the lincoln aod's wont fit.
Have fun with it. You'll learn stuff and save money and gain bragging rights.