Speedo Is off, Transmission shirts early

Ok So, I am a Mopar guy. For some reason i keep aquiring Chevy's. Life is tough. I have a 1994 Z28 383 roller nitro motor, 4l60E, posi rear 3:73's with 17" corvette wheels. My speedo is off by anywhere from 10-30mph depending on speed of course, the transmission is in 4th gear by the time the speedo reaches 45mph on the gauge (35mph actual). I called the dealer ship to see if they could recalibrate the darn thing and they basically shrugged their shoulders and said that they had no clue and then another said that it is not even possible. So here I am on the forum for Camaro's because I know someone out there has modified one of these cars and had the same problem. Also i don't know if it matters but there is a check light on the dash that looks like 3/4's of a box w/ squiggly lines and a downward arrow in the center of it. The car has all factory working intruments. The TEMP gauge does work properly, and all fluids have been flushed and filled.