Steve, the self-exciting is usually referred to as a 'one wire' and the non self-exciting is usually referred to as a 'three wire'.
One wire alternators are a bit easier to install (two less wires!). Hot rodders have used them for many years.
What are the two extra wires for on the three wire? One is the exciter wire which 'excites' the alternator when the ignition key is turned on. The second is a remote voltage sensing wire for the internal voltage regulator.
The three wire is almost just as easy to install. It has three advantages. One is that you can run an idiot light to your dash if you want to. The second is that it senses the load at the main electrical junction terminal instead of at the alternator. The third is that it is activated (excited) when the ignition is turned on as compared to having to rely on the engine to reach a certain RPM.
I prefer the three wire because I run A/C in my truck and I want the voltage regulator to know the load at the junction point, but that's a personal preference.
As others have stated, the lower capacity alternator should work fine unless you plan on adding several additional heavy power draw accessories. I run a 10SI three wire 63 amp on my '57 with a 235 and A/C. Works fine.
Both work fine. Mostly personal preference.
Matt
