SparkFun Electronics
This link was from a few posts back.
I would really like a serial version, but if you are going to a strictly USB version, I could most likely use my computer at work to flash the chip. Ill trust you in what you recommend is going to work best.
I did some reading through my 80s and early 90s GM factory service manuals. From what I can gather, it appears there are a few different combinations, but all with the same result. 1- Cable driven speedos have an optical pickup/module as a VSS, which sends 2000 PPM (pulses per mile) to the ECM. 2- Electronic speedos with non-electronic trannys, have a VSS buffer between the VSS and ECM, and it sends 2000 PPM to the ECM, and 4000 PPM to the gauge cluster. 3- Electronic speedos with electronic controlled trannys go direct to the ECM, and the ECM sends 4000 PPM to the gauge cluster.