The only issue with any 727 small block hi-torque starter when fitted to a G3 is the clearancing between the pan rail and the body of the starter.
The pan rail has two protruding bolt holes on each side where the larger factory pan bolts locate into the rail.
In normal circumstances these are just ground back to about half size to allow for fitting of the starter.(when using the Milodon pan)
So....as long as the dimensions are similar between starters I cant see why you'd have an issue.
Maybe call them back and ask for the circumference of the body of both......thatll tell you if there's likely to be a hitch.