I had that issue with parts store t-stats, all I did was take my dremel and open up the hole just a tiny bit and it slipped right in. The seal is on the flat face so opening the hole a little wont affect the seal of it. (this is the inner hole that has the sealing surface you're talking about right? or are you talking about it not fitting into the housing at all?) either way depending on how much of a size difference if it is just a hair that needs to fit you can either open up the housing, shrink down the t-stat, or try a different brand t-stat...
'94 2.3L, M5OD, BW1354 manual, 4x4, 31x10.5 BFG A/Ts, 15x8 AR Chamber teflon rims, 7.5 4.10 lock-right w/disks, D44 SAS 4.10 aussie-locker, duff traction bars, electric fans, spintech muffler, duff poly body mount bushings, Lightforce RMDL140's, cobra 18wxst II, herculined floor, 375,XXX
'87 camaro 305 carbed (needs engine)
'92 subaru legacy 2.2L Turbo (needs tranny)
TRS Tech Support Staff