Roger,
Good advice. On the long run a new gas tank is certainly the best solution.
As to non-fitting used tanks...
there are no basic differences between Euro and US tanks or the sheet metal per se (with the exception of the side marker lenses where applicable).
Most likely you got a used one that was slightly dammaged (e.g. hit from behind or so) which is why it did not fit correctly.
There are only three tanks for the W113 - not more.
1) Early 230 SLs up to serial 000202 - 65 liters (different position of the outgoing lines)
2) all other 230 SLs - also 65 liters
3) 250-280 SL gas tank - 82 liters
The latter fits also for 230ies without sparewheel well.
One additional possibility to get out the dirt, rust or sealant from the flower pot more easily is to cut in a servicing opening (e.g. where the tank sending unit sits) so that you can access the inside easier than through the pretty narrow opening of the tank sending unit.
After cleaning the opening needs to be (welded - if no gas fumes are aroung anymore) or closed by a kind of lid which you can built from the cutted-out opening.
A handicraft job.
Good luck
Achim