That's a tough one. From the picture it would seem that there is a threaded boss welded to the bottom of the tank. It looks like the pot metal threads of the bowl broke off and left part of it inside the boss. It also looks like they used some sort of thread sealant on the joint.
It would seem most likely that the sealant set up and became stronger than the metal of the bowl. Any sealant that is impervious to diesel is most likely impervious to most solvents except perhaps alcohol. If I remember right, Permetex #2 was solved with denatured alcohol, but it was a dark brown, almost black compound. This looks white, which would suggest a teflon past of some sort.
I would try an easy out and use a heat gun to get that flange as hot as possible. That will do two things, expand the flange and soften the sealant.
The threads on the bottom of the tank in my 2006 284 were threaded 1/4" pipe thread, but Jinma is known to have changed the tank designs in that era and yours may well be straight threads or some metric design. If they are pipe threads, getting any movement should cause them to loosen with just a partial turn as they are tapered threads.
I don't know of any way to get the tank out without removing the hood and instrument cowling. An ugly job for sure, but it can be done. You might be able to remove all the cowl mounting bolts and get it to lift up enough to clear the filler neck. The wiring harness should have two large connectors between the instrument cluster and the main harness. You may be able to just unplug them to get enough movement to clear the filler neck. You may have to remove the steering wheel first. It is all a tight fit.
Another idea besides the easy out is a left hand drill bit, but you run the risk of damaging the threads if you don't get one the right size. Heat is probably your best friend on this one.
Let us know how you make out.
