I was never satisfied with using a 26 r battery because it had only the mounting ledge on the long sides of the battery. The short sides have no ledge. A tii battery tray was never originally drilled for a hold down on the side but on the end of the battery tray. I tried group 48 but it would not fit especially with the Alpina air box. Other members had mentioned a Mercedes starter battery BCI group V4. It has ledges on both the long and short sides of the battery. But it is about an inch short which was easily fixed with a spacer, in my case a piece garage door gasket hidden well from view. The V4 is substantially shorter in height which made it easier to install.
On sale the price was similar to a high end 26R.
One caveat; I don’t use my car in the winter and have no big power draining accessories.