Thanks all...I have the new tank and will be painting the outside with hammerite. I'm not going to coat the inside I've heard too many problems with paint clogging the fuel. If I have to replace the tank in another 30 years I won't mind !
I'm going to paint as much of the underneath as possible. Paying special attention to the area above the chassis as mentioned here. Hopefully nothing needs welding but until I really clean the rust away I won't know for sure.
Might slap some paint on the axle as well. The Diff cover is a bit rusty so will be nice to get that looking better.
Again thanks for the advice...some clean looking undersides out there !
If it isn't broken, I can still fix it