I went to the web site and it was a little confusing. It says the tank is for a Toyota but the category is Ford. It would be nice if there is more detailed information on the site.
Around 1995, the PO of the Karma Cruiser had me replace the glass runs and perimeter weatherstripping on the front doors. Like most early 40s, the lower door skins had stress cracks at the pressure points where the tracks from the upper door...
I'd be looking for another gear and the one it runs with. You have it apart, fix it correctly and be done. Otherwise it could break at any time and take out more stuff and perhaps strand the rig.
Maybe I'm missing something, but it doesn't look that bad to me.
Rather than cutting and welding the frame, which I think would involve fish plating the repair (which makes it obvious the frame was repaired) why not take the frame to a shop...