I have a '66 230sl with the "lay flat" spare but it has the smaller, 230sl tank. The car had been sitting for quite some time when I got it and the tank had an enormous amount of crud in it. I decided it was better to replace the tank than to try to have it cleaned out and refinished. The larger tank is not an easy task without making modifications. I'm not saying it can't be done, but it's not easy. We tried, but it wan't going to fit without some modifications - it's not a simple "bolt on" process. I ended up waiting for the next round of new (and smaller) tanks from MB so that it would just bolt back on, plus I really wanted to keep the car as original as possible. I'm the third owner and just now beginning the process of getting its first repaint.
My understanding, and members please correct me if I am wrong, is that the later model 230's were built alongside the 250's in '66. The 250's came with the larger tank, but I don't think any of the 230's had it.