Mugsy,
I was thinking about doing a 5 SPD (overdrive) myself (68 c-10, 250 CID, 3.90 (i think)rear). There are many posts available on how to do the conversion, just use the search engine above. I believe the tranny of choice was a T-5, from an S-10, but it does require some machine work and figuring out a good clutch plate. An article on this conversion is in the Tech Tips at the top of the page.
Possible consideration: In Vegas, there is a shop that makes overdrive units to place on SM 420, and SM 465 trannys. They are pricey, about $2,000, but they'll let you keep all the power you have with that low 1st gear. Since I do some heavy work occasionally, that's the route I've chosen. Use the Google search engine to find the shop, or I can e-mail you the link.
