Nantucket...Im here on Long Island, and although I swapped out the 3.73's at Classic Concepts ( the local Superformance Installer) there were plenty of speed shops to handle the swap. Look for shops that build roadrace/dragrace cars or even shops that work on late model muscle cars like mustangs and such as there constantly changing gear ratios on street cars.
Do you know which Tremec transmission you have ????
Im guessing since your SPF car # is in the 1300's like mine it originally had a Tremec TKO with a 3.27 first gear,....was in the same boat, big block with 3.73's rear gears and a 3.27 first gear in the trans. First gear was totally useless and annoying to drive in traffic. The dana rear came out around car # 2000...so like mine you should have the ford 8.8 rear.
You might want to try the Second Strike gearing calculator first to see the difference in ratios to actual speed. I say this because...if you have the trans with the 3.27 first gear....changing the rear end ratio from 3.73 to lets say a Ford motorsport 3.55 or 3.27 will not drastically change the annoying driveability of a useless first gear.
http://www.secondstrike.com/Technical/GearCalc.asp
I know that first hand as I also changed transmissions to a TKO 600 with a 2.87 first gear....that and the 3.27's in the rear made it a pleasure to drive.