I had a small patch remaining from the old pattern for the fuel tank hole but it was filled up mostly with weld. I had a go at making something of it but it ended up looking awful (not that you will see it anyway but I will know it's there!). Anyway, I used the old one as a pattern for the size of hole and dimensions and cut a new piece to fit. I will weld it in from underneath and follow the pattern of the corrugation to plug the ends of the floor.
I still need to trial fit the fuel tank and make a couple of cradles up for it to sit on as the old ones were completely shot. I think it needs lifted by about 1cm, maybe less, but once I get the tank in I can have a look see. In the top photo you can just make out the seal which sits between the tank and the floor.
I have a bay window tank lurking around the garage as well somewhere. I fitted one of these to my previous split and it was useful having the extra capacity. Not sure if I'll go down that route again though as I had endless problems with the fuel sloshing out and also it taking ages to fill despite wiring up breathers and repositioning the fuel filler tube several times.