I suggest you mount the seat on any pedestal, if that's what you want, and make sure they clear turning before drilling holes, don't just go by the seat pedestal or tracks. Underneath I used two washers, one larger on first then a smaller washer that fits snug to the bolt, worked fine and was secure. In mine I had shoulder belt mounts, usually depends on year, even the utility van had the mount holes, just not belts. If you build it as an RV camper, you probably won't need belts, check locally, but would be safer to have them. The floor may already have flat spots where seats mount, cheaper to build off the same body. Good idea, build it like you want it!
Oh, you can get bench seats with shoulder restraints, check the salvage yards and the custom van shops as they take them out for conversions, handicap conversions too.
