alright, sounds like the gears, for sure since its a stick, were installed incorrectly. I would suggest a set of 3.73 if you want some performance, or the stock 2,73 or 3.55 ratio, proably the 2.73 as long as it is a 7.5. The 2.73 gears will set the speedo correctly once installed, and will need to be done professionally, as will any gear, and run around 700 total. You may also want to invest in a traction lock if there isnt one already. If you want a bit more performance get the 3.73s but you will need a retune to set the odometer back
Thus the 2.73 gears on the stock 7.5 rear will be your best bet. Also will be able to stay in 1st a bit longer, as the 4.10s basically dont have a first it hits 5k rpm or so after letting of the clutch. You can always just start in 2nd and see if the issue persists, but if the gears wernt properly installed they will eventually fail.
I do NOT know how to tell the difference,between 7.5 and 8.8, unfortunately.
As for the AC you will want to look inside the dash to see if the components are there, If everything is in the dash that needs to be there, and all the components are in the engine comparment you probably just need to put hoses and frion,
Unfortunatly im not very knowledgable on the AC system.
2003 Redfire V6,
Show: Smoked everything, cobra front bumper, gt rear bumper, rear decklid and lower valence blackout with black mustang letters, 5% rear front 15%, black 98 Cobra rims front, 95 Cobra R rear black with chrome lip, black scoops and spoiler and exhaust tips
Go: CAI, True Duals with flowmaster super 44's, Bama Tune, full suspension (lowered, 8.8, 4.10s, UCA, LCA, springs, CC plates)