Thanks Shielsy. In the end I wussed out and cut the end out of the existing one. Its not resto quality, but it's also no longer a rusty hole. I'll try and get pictures tomorrow.
I think my goal for now is to get it solid and MOTable then enjoy it for a couple of years while I save up and then take it off the road again and properly go through it. Take the body off the pan and go to town on it all.
There's a lot of bits that are solid, but not very pretty. Plus I would like to do some more cosmetic things like getting rid of the fuel flap on the front quarter.