Those things are unibody, so the load floor structure gives it strength. I remember a similar cutaway camper conversion done on a 1st generation Econoline. The Econoline had tandem rear axles (1 wheel behind the other; not dual tires on 1 axle).

A 62 Corvair would have a 145 cubic inch air cooled flat six; 1964 they increased to 164 cubic inches.