Its not really fare to compare a early cummins(89-93) to a 6.9(83-87) the 6.9 was gone before the cummins even came out.
plus Motor wise yeah the cummins is superior. but Everything else on that dodge chassis is junk. I'd buy the ford any day for that reason. If you want a cummins stick it in a ford.
as far as the 6.9 to 6.0 comparason I know I seen some pictures of a twin turbo IDI That would proably do 14s.
I don't know about that I test drove a 6.0 in 03 and my 92 7.3 IDI wasn't far off from it. the big difference between the trucks was it was an automatic and mine was a stick I had 4.10s it had 3.73s..... oh yeah it had a turbo I didn't.