Well I am no expert on painting (since I have yet to do any myself, so you were warned
), but if I didn't care about painting the underbelly I would just glue the body unto a used up gift card before painting and placing base material (whether it is fully assembled or not before this seems like a personal choice to me) and than paint (that way maybe I can use the basing material to cover up any paint job sins), alternatively you might want to paint a bit of the front ribs etc. first and leave the middle front rib cage, belly, and crouch unpainted for gluing wmyers since you strike me as a detail oriented fellow.
However, if I were painting the belly I would either not bother with a base or buy one of those see through bases (don't even bother with base décor
) and ask around hobby shops (the model-diorama ones) what the best see through glue is for the job (I recommend explaining why you want it first, perhaps even bringing the base and figure along) same goes for paint to paint glue jobs.