Duracell 8D's?

[krush]

Your engines can start with a group 31. Cummins did a good job of keeping the required amperage low. My Detroit's won't start on single 31.

Is it feasible to replace the 8D's with two smaller batteries that will fit in the boxes and have equal starting amps?

I would not replace the 8d with one group 31. I'd use at least two batteries. The internet says three group 24 (and maybe 34) fit in 8d space.

My RV has same engine and two batteries in parallel to start it. It also has 2 8d gels for the house, but they are on a slide out tray and much easier to get to.