Well Since i've been told nothing from any other ford engine will interchange with a 351m/400 and i wasn't sure if the g3 was being used when the 351m400 were in production..
The alt came out of a first gen Taurus with the 3.8. $30 at a local junkyard. I had it tested and it was at about 140 amps. A little better than the stocker! With the extra lights I have and the big amp that I run it was a worth while upgrade. And not to hard on the wallet either.
The "lock bolt" that holds the alt in position to keep belt tension can be an issue. Make sure you get an alt that has threads in that hole. I makes adjusting the belt a lot easier if you don't have to run two wrenches. One to hold the bolt and one to tighten the nut, all while trying to hold tension on the alt to keep the belt tight.
I also "clocked" the rear portion of the 3g. All of the wires and terminals where at what is now the top portion of the alt. Not too pretty looking with all the wires. Just pull the case half bolts out and turn the rear half where you want it and put the bolts back in. Just don't pull the halves apart, only rotate them.