The parallel frame rails of the S10 make it a great candidate for lengthening. I don't recall my exact measurements, but there's a standard C-channel hat fits inside the S10 frame really well.
I had to add 17-1/2 inches to my S10 longbed frame and it was the easiest part of my project.
See page: http://www.unkamike.com/truck/p2page4.htm
The c-channel fit so snugly inside the frame rail, that I was able to slide things like a big car antenna to lengthen and shorten it until the tires were centered in the wheel wells where I wanted them.
At that point, I put some small machine screws in each side to hold it square until it could be welded.
Mike V

46 Chev One Ton Panel - S10 Frame Project

"What if the Hokey-Pokey is REALLY what it's all about?" - J. Buffet