My old van (late model) had side pans and was a twin slider and a bolt on centre pan. I used to get any rusty bits patched up along with the outer sills. One time I bought some new side pans on an offer so cut the old ones off to replace them. I ended up spending a few months replacing almost everything back to the central chassis rails 
I bought a late high top on eBay unsighted one night , it had a mot so went to collect it and drive it 80 miles home. Amongst other horrors, the underside had a one piece belly pan in place that was a plain sheet of steel that went from one sill to the other and was vaugley curved to fit. The wheels all came loose after a few miles but it got home ok …. sold it on but never disturbed the big belly pan
I bought a late high top on eBay unsighted one night , it had a mot so went to collect it and drive it 80 miles home. Amongst other horrors, the underside had a one piece belly pan in place that was a plain sheet of steel that went from one sill to the other and was vaugley curved to fit. The wheels all came loose after a few miles but it got home ok …. sold it on but never disturbed the big belly pan