Seriously for a horizontal shaft motor project buy a motor that is already a horizontal shaft, they are out there and if you go looking enough they are cheap, there is a 8hp I/C horizontal briggs local to me asking $50, I bought my 11hp horizontal motor for $20, its not worth the stuffing about trying to convert it.
4hp into two 40amp alternators might be pushing it, off the top of my head I cant remember how much hp one would need at full load, but you can hear the idle of my old corolla drop just by turning the headlights on.
