Felt a bit bad your post went unanswered VB so here's an idea or two........
I've not seen a front bench seat in any domestic van built in the past 50 years so that says something. The engines have always been place where their intrusion into the front seating area is unavoidable. Anyone here remember those huge doghouses smack dab in between the front seats?
Rather than modifying the van to accommodate a full-time front bench seat consider the typical Ford center console does remove which gains 8"-12" of space---naturally the engine cover can't be removed and the van still driven.
With the center console removed some sort of "bridge" between the front seats could be made and installed when the extra sleeping area is needed. You'd still have the steering wheel in place, a tilt function helping a bit.
That's all I've got--not much but maybe helpful in your creative process?