More from the reference library.
Marre and Pinkepank op. cit. (as us pedants like to say), page 310, has two photos of cab-challenged Missouri Pacific units. One is another photo of U30C in cabless form, the other is B23-7 (mis-identified as a B30-7A) 4607 with its... angular... cab. The caption to that one says that 4607 "received a homemade cab that looks more EMD than GE."
If that's right, and the cab was fabricated in the railroad's own shops, it answers a number of questions: like "How did they get it to mate neatly with a GE nose and long hood?" It also suggests an answer to the question of why this design rather than one looking more like the original GE cab. The cabs on GE U-series and Dash-7 locomotives, with their curved roof profile, would almost certainly have been harder to reproduce than this angular design. (Making curved pieces of steel out of the flat plates you buy from the steel mill takes specialized machinery. A steam era railroad shop might have had it-- boilers, after all, are made out of curved sheets-- but a diesel era shop might not have. When GE took over the former Erie Railroad shops in Hornell, NY, they found that the shop still had the boiler-plate-curving equipment, which they found very useful in their locomotive rebuilding work.)
-- Note that GE itself abandoned the curved roofline in favour of a "Spartan-ish" cab design when they introduced the B39-8E and C39-8E models in 1987: I think in a move to reduce fabrication costs.
As for the different noses on the API620 in the link from the original post in this string and the link MEC 407 has just provided... I suspect the noses of these units were fabricated in the F del P shops, and that almost every unit would have been a bit different from the one before. I would think the "Century-inspired" nose would be easier to build than the "GE-inspired" one (front plate vertical instead of sloped, etc). I note that the API620 with the Century-inspired nose has a higher number than the one with the GE-inspired nose: I think there may be a general tendency for aesthetic refinements to be dropped in favour of simpler, easier-to-make, features as a program goes on (compare, for instance, early and late Pennsylvania Railroad T1 steamers).