www.e-zchassisswaps.com
Looks like their products are well thought out and fabricated.
The idea of building a rod with your second truck seems to be a sound one. On the other hand, I would advise against trying to build a blower motor "on the cheap". I don't have ANY experience with them, but I highly doubt that a "farmer rebuild" would handle the extra stresses that a blower will add to it-not to mention, passing up on proper machining will not allow the cylinders to seal properly thus reducing the amount of boost you actually achieve.
There's no way on earth a 7.5" or 7.65" ring gear S-10 diff will stand up to a blown motor...I doubt an 8.5" ring gear S-10 diff would live long behind one, either. A 9" Ford, set up to bolt in would be the best diff for that kind of application.
The matter of wheelbase is entirely up to you. In most cases, the most "correct" looking fit for an early truck would be the regular cab longbox S-10 chassis, which runs 117.9" wheelbase. Some of the guys building Chevy Advance Design pickups use the longer extended cab shortbox which has a 122.9" wheelbase and allows the use of a long box (3/4 ton pickup). I've seen one or two done using the regular cab shortbox chassis, which has a 108" wheelbase. If you plan to run a flatbed, or a shortened pickup bed, this might just look okay. I would stay away from the Blazer/Jimmy chassis, as the two door only has a 98" wheelbase. The four door has a 108" wheelbase, BUT the kickup over the rear axle is higher than the equivalent pickup chassis.
WHEW! My fingers are tired!
That being said, I'd like to offer one more link:
www.s-series.org
A great place to get platform specific info if you do decide to use an S-truck chassis. I'm a member over there, as well (see my avitar pic)
Hope some of this info helps. Keep us posted once you get going