I wish I would have known how poorly the repro sending units were before I wasted good money on one. I managed to find a NOS unit at Carlisle last year. It reguired only minimal adjustment and read perfectly on my gauge.
I would have had my original rebuilt if I couldn't find a NOS. Here are 2 places on the web that I found. I have no information regarding their quality of service.
http://www.bobsspeedometer.com/1/120/services.asphttp://tristarrradiator.com/Classic%20%20Vintage%20Page/Classic%20%20Vintage%20Page.html