Get an ambulance body, bosh a few holes in it, bolt a park bench in the back, job done! Rear body is a fibreglass drop-on to the load bed, slide it off and you've got a straight pinz.
The hard-top version (712K) already has a few extra seats I think, but they're hard to find as they weren't popular with the military.
Bear in mind the 16MPG fuel figures, not sure if you can LPG it but I expect so (although I won't).