you could unpick the welds holding the steel ribbing to the sheet metal, then use the ribbing with the fibreglass skin you just made, and add another layer of resin to hold the two together from the underside?
I'd have to agree, probably a better idea to make a basic mould first and then go from there building up vents etc. But as long as you keep it to a couple of layers thick you should be fine though. The difference will be negligible.