I'd actually guess that making a full EV first is probably the easiest way to go.
(See a '65 Mustang EV here:
http://www.austinev.org/evalbum/098.html )
Easier as it has been done before, so plenty help from friendly enthusiasts and a wealth of experience out there in simple EV conversions.
Then to make it a hybrid, you should just add a small generator in the boot (maybe a Honda 30hp?). With that arrangement you'd actually have a more advanced and efficient powertrain than that in existing hybrids (
ie a plug-in hybrid).
Much easier doing it this way as it avoids a lot of complicated software / integration problems that you'd have with a conventional small battery hybrid system (
eg trying to integrate engine / motor output shafts onto the same differential etc).