If your ABS light is coming on it sounds like they didn't put the proper reluctor in the rear end when they did the gear change. As far as tuning the speedometer, if you have any car friends with a programmer it should be able to change the speedo settings as its a pretty basic change to the pcm. Also any basic performance shop in your area should have an OBD2 reader and programmer, they'd probably be able to help you out.
1968 C10: Bagged and Bodied with a 305/350
1994 9c1: Stock...for now
1995 BBB: Stock-ish
1996 B4U: Parts car
1996 DCM: Modded