Went from 06 2500 Duramax to 14 Ram 2500 (Hemi 6.4). If I remember correctly the bed height of the Ram is about 2-3 inches taller. I've talked to someone who tried a Rambox with a fiver and it didn't work so check that out if you're thinking of that combo.
The megacabs are nice but are becoming rare since Ram started making a realistic 4 door. My 2014 CC has considerably more room than my 06 Dmax (I'm guessing your 07 is the classic body style thus the same as my 06?).
The Rams are super nice trucks and I'd give my highest recommendation without hesitating. I'm not much help on the Cummins/Aisin thing since I'm a gasser now but if you're even slightly considering gas go test drive the 6.4 Hemi. 410HP/430TQ is nearly identical to my Dmax @ 360HP/650TQ. The diesel wins in the mountains but not by a lot and the Hemi as a daily driver is awesome. To be fair though any brand 2014 is probably a better DD just because it's 8 years newer technology and not 8 years worn out.