Don, Carl's idea of modifying his DC (and the results was it looked much like a D) came from out dad. Dad's old C, as we called it, was actually a modified CC.
I explained our logic, to Chuck Machinist far below in a reply to him, for switching to a wide front axle. The reason was it made the steering wheel much more user-friendly. The C was not needed for ROW CROP cultivating etc. like the CC could be used for.
The other main reason was to have a tillage speed between the DC's 2nd and 3rd gears. Keeping the DC’s larger diameter axle chain sprockets and switching to smaller diameter tires, Carl’s “D” could now easily pull 3 bottoms in the slower 3rd gear but faster than the DC's 2nd gear.
Like father like son, Carl's D was not used for row crop operations like the DC was made to also do. Like dad, Carl also had a B John Deere as a companion tractor to his "D". Mowing, raking, etc. did not need brute horsepower. (Close your eyed and ears lol---The B john Deere was a nimble tractor with good brakes and easy steering and 6 speeds!!!!)
The D series with big AL pistons and 5505 high compression head had brute horsepower. Also, the AL pistons allowed the D series to smoothly run around 1500 rpm like the M Farmall/400 did. Carl did tillage for 10 years with the rpm jacked to around 1500 rpm!!! During refurbishing when I inspected the bottom end, I expected the Babbitt to be shot. Only the front con rod needed replacing.
The other DCs, that I refurbished could pulled 3 bottoms easy in 2nd gear but struggled a lot in 3rd gear. All had 4 or 4.06 bore AL pistons and the 5505 high compression.