I actually had a set of brackets fabbed up for my 83 c10 to add a second alt under the stock passenger side alt (where the AIR pump would be), but left them in a warehouse I shared workspace in and I think they went in the scrap pile, never to be seen again. At a minimum, comfort with a GOOD welder and metal shaping skills are a requirement, and significant time to spend shimming and adjusting for belt angles. Wiring is relatively easy, especially if you use a 1 wire alt for your second alternator.
To answer Irish's question of why:
In my case, I intended to isolate a secondary electrical system to support a potentially LARGE surge load. Namely, cb/ham radio amplifiers I was really thinking about getting into, auxiliary lights, stereo amp, etc. A relatively small amp for CB or Ham radio use can EASILY exceed the charging capability of a 140 amp alt even when it is the only load, though that largely depends on how much you flap your jaws with the key down. During key down competitions, its not uncommon to see daily driven vehicles with a single stock alt for one electrical system, and 4 or more 300+ amp alts on another circuit. The reason being, many alts for these bands are rated at 12v, and 14.4v from an alternator(s) capable of keeping up with charging 10+ batteries provides a little extra kick.
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