The standard fuel tank mounts by way of two circular rubber fittings that slide into two curved metal cups that are spot welded to the neck gusset as shown in the photo below.

I intend to use a Sportster tank that I have acquired and the front mounting brackets line up exactly where the steel cups are positioned. Using adjustable grips, a large wrench and hammer and chisel I carefully worked my way around the offending metal until, albeit in several pieces, each cup was prised free of the frame.  Left behind was the ugly remains of the spot welds and small lumps of attached metal.

At first I used a file to reduce the blobs of weld but because of the surrounding framework it proved impossible to keep the file flat against the neck gusset. I was forced to use an angle grinder to take off the remaining steel. I'm not concerned about getting a perfect finish as yet because the area being worked will need to be drilled through and a metal tube welded inside the gusset to provide a strong mount for the Sportster tank. I'll worry about making it pretty after all the modifications have been completed.  I sprayed on a  generous coat of primer to keep the dreaded rust at bay. The next time I do any work to the frame should be after Mr Sooty has completed the frame rail and swing arm modifications.