he already has 4.10s.
I'd look into LCA's, Plenum/TB, Cams (cams alone + install, there's your $1k and then some) or Longtubes. You could get any combo of LT's, LCAs and TB/Plenum (2 of the 3, all 3 if you got cheaper stuff) but cams its either all or nothing.
You're about where Im at in modding and Im stuck in a rut too. Not sure if I want to throw more money at the stang just so I can run low 13s, or put it back to stock and buy a Mach 1 (the exhaust and gears will swap over, the rest I'd sell.)
Its a tough call, save a few grand for a blower or cut your losses and uprgrade to a better starting platform. I know the Mach would be another money pit, any car I buy would. But instead of spending a lot of money to go 12s, Id spend a lot to go 11s.