Welcome.... Love those 67-68 stepsides. I sho would like to find one in good condition here in Texas.
Re: speed off. My 68 was off also and I went to the dealer and bought a driven gear (didn't have to change the drive gear). I used one with less teeth than what I had to increase the cable rpms. Luckily I didn't have to do more than that.
Do a search above and you should find several threads on it. I used "driven gear" but you can play with it to see what comes up. Here's a couple of sites.
http://www.transmissioncenter.net/sp...n_______va.htm
http://www.bgsoflex.com/speedo1.html