I'm running a GM 10si alternator in one-wire configuration on my 261 and it is working just fine. It bolted right up by just elongating the holes in the bottom generator bracket and flipping over the top bracket. I plan to switch it to 3-wire eventually, but it works just fine as-is. It starts charging as soon as you bump the engine over 1000 rpm, and it still charges at 600 rpm with the headlights on. I just ran a new #10 wire up to the post on the ammeter that connects to the voltage regulator and disconnected the old voltage regulator. The 63 amp 10si works just fine for my truck. Any of the GM si series alternators are good and dirt cheap. Mine was $30 at autozone.
I have also run one on a wide-belt 216 by just swapping the old generator pulley over to the alternator. The shaft diameter was exactly the same.
