My generator swap plan was derailed tonight.
I have been told and have read many times that all you have to do to go between a 19 and 26 tooth generator was to turn the idler gear eccentric pin. I do not see the point when the turned boss, or land on the generators are two different diameters. The 19 A generator is much larger (4") than the 40Amp generator (3 1/4). I hit the books tonight but came up empty looking for my generator housing assmy. 4F6979 with the small 3 1/4" hole and what seems to be a 40A generator. In the books I see 4F6978 with the 19A generator, 4F6980 with the 6V 40A generator, and 24V generator. Naturally the tag is gone from my Generator. I am hoping someone here can identify what I have from the pic and the 4F6979 housing number. Can someone confirm the generator in the pic as Delco Remy or American Bosch. My goal is to rebuild with 12 volt fields. My second option is to chuck up the generator body up in the lathe and shorten the body to accept an adapter ring, love joy coupling, and a 12V alternator. Your help will be greatly appreciated.   
 

4F6979 crosses to the 4F6980, used in a variety of generator groups, for both the 6V and 24V.

im in the process of doing a conversion myself. how im doing it is after stripping down the generator i had a new longer shaft made to take the original gear, fitted new bearings along with a locking collar and a taper-lock pulley to the shaft all mounted in the original housing. this will drive a 10si delco alternator mounted to a bracket i bolted to the generator housing
this will give me the required speed to properly drive the alternator with tension set by a slide bracket off the engine block.(be sure to order one with the right direction fan the internals don't care its just the fan works better one way than the other)

if you remove the fan and brushes there might be just enough length to put one half of a lovejoy or fenner coupler on the original shaft. and save having to make or a new shaft.

We’re doing a engine swap on our D47U by taking a good 315 power unit that came out of a Bucyrus-Erie shovel.
I’m going to use the 40amp generator like you show.
When pulling apart I noticed the different gear diameters.
 

We’re doing a engine swap on our D47U by taking a good 315 power unit that came out of a Bucyrus-Erie shovel.<br />I’m going to use the 40amp generator like you show.<br />When pulling apart I noticed the different gear diameters.<br /> 

It appears that, depending on which generator you have, you can run both the 24V or 6V generators with the small 19t gear and 17T idler and the 4F6979-4F6980 housing, or run the 6V generator with the larger 26T gear and the 4F6978 housing, no idler.

Your dealing with an American Bosch Generator. You've been given some bogus information. The sloted mounting bolt locations are solely used for adjusting back lash of the gears. Has nothing to do with changing gear diameters.
Be aware that adding an idler gear changes rotation.

Thanks for all of the info. Today I found out that the 6V American Bosch still works. I don't have the heart to chop up a working generator and finding 12 or 24V fields seems to be an issue, so on the shelf it goes for a future project. I am now beating the bushes for a dead core to cut down and mount an alternator to. My other D6 has a alternator mounted to the top of the generator with a belt drive. It works well. My plan is to build my own version of the $900.00 mount as sold by Walmart. I plan to use a Love Joy coupling. The alternator will get a new fan blowing the correct direction. The pulley will be replaced with a spacer and the nut will be replaced with a love joy that will have two flats milled so a wrench will fit and the center threaded match the alternator. I plan to shorten the generator body and reinstall the rear cover with the shaft exposed. Reason being is I want bearings on each end of the driven shaft to provide support for the gear to prevent linear and axial thrust alignment issues under load. I will work out the fine points just as soon as I get a core apart on the bench. 

wanting the shaft supported at both ends is why i made a longer shaft it also meant i didn't have to modify the generator housing other than drill a second hole to mount the alternator bracket. the original armature when you remove the fan protrudes just over half inch from the end plate might be enough to support one half of the coupler. or simply make a longer shaft the limiting factor is overall length and clearing the oil filter body. the bearings are the same both ends 6023-2RS one of the most common sizes in the world and cost about 10 bucks each.
if you shorten the barrel itself you keep the end plates as factory and thus the bearings are as original. there is a spring steel thrust washer in the original set up to set the end play. to have this work a locking collar against the gear end bearing like i used (was cheaper and simpler than a stepped shaft) the collar locates on a dimple drilled in the shaft thus holding the whole assembly in place. making a longer shaft means it could be restored to original if i ever wanted as the housing is untouched except for a extra bolt hole. but in my case the unit was no functioning so it wont happen.

or simply make a stepped shaft you only need the shaft to be 15 thou larger between the bearing areas and the shaft will self centre between the bearings