Thank you, Scot, for the gallery of weird and wonderfuls!
The CP chopnose RS-3 has to be in the running for "ugliest diesel locomotive of all time"! The first of Conrail's RS-3M (RS-3 re-engined with EMD 12-567 engine: the first one was actually done before the beginning of Conrail by either PC or EL) had the whole short hood removed, so it looked like an end-cab switcher, but with a back porch.
As for the two Alco RS-3 with EMD GP-style long hoods, these were re-engining jobs. When a (hood-type) locomotive was re-engined in a railroad's own shops, the new engine was often housed in the original hood: a tight squeeze in some cases, necessitating modifications: many Conrail RS-3M had boxy structures in the middle of the long hood because the original Alco hood didn't have enough clearance for power-assembly changes on the EMD engine. ... When a railroad sent a locomotive to a builder force-engining, the builder would often remove the original long hood entirely and cover it's new engine with one of their own hoods, as with these two. (An EMD long hood would probably be easier to fit an EMD radiator into, so the EMD factory staff didn't have to do creative plumbing.) ... The Nickel Plate had four Baldwin road switchers, which they sent off-- two to Alco, two to EMD-- for re-engining: they came back with GP-9 or RS-11 long hoods.
The Alco C-628 with the GE cab is most relevant to this string. GE has, over the years, done a lot of locomotive modification and repair work (often at a plant in Cleveland, OH). This photo is dated 1973, by which time Alco was no longer around to provide a replacement cab. So GE Cleveland (I'm guessing) ordered a cab from GE Erie.
Just WHY Missouri Pacific put EMD cabs on its damaged GE units I don't know (though my guess would be price), but I can fill in a bit of the history.
"Extra 2200 South" issue 56 (official date April-June 1976, though it probably came out later), page 9, has a photo (dated26.vi.1976) of MP U30C with no nose or cab: just a flat sheet closing off the front of the long hood. Caption says "U30C #3319was outshopped 6/24/76with damaged cab removed and hood sealed so it can operate as a "B-unit" until new cab arrives."
"Extra 2200 South" issue 58 ("October-December 1976"), page 9, has a photo of U30C 3311 with EMD cab (but GE nose): caption says "Fresh from shop, rebuilt due to wreck at Meeker, LA, is U30C 3311 at North Little Rock, AR, 11/12/76."
"Extra 2200 South" issue 60 ("April-June 1977"), page 11, has a photo, dated 8.v.77, of U23B 2256 with EMD cab and GE nose. Photo caption mentions that 2282 had "recently" received a similar treatment, and "Also temporarily cabless U30C 3319 showed up about the same time with anew GE cab." News item on same page doesn't add any further details.
So it seems that MP's "adventure" of putting EMD cabs on damaged GE locomotives lasted a few months, in late 1976 and early 1977.