I hate to tell you this, SOU2645, but your information on the RS-3 251 repowerings is incorrect. All three units did have RS-11 hoods.
The L&N unit, 1350, was rebuilt with an RS-11 hood, per an L&N drawing that appears in George Elwood's excellent Fallen Flags site. Unfortunately there is no photo on that site, but I have seen one in a book on the L&N that one of my associates has in his collection. This same gentleman also has some Alco engineering drawings that pertain to this particular unit and it's new engine.
The C&NW units, 1613 and 1624, were wrecked in a derailment at New Ulm, NM in August, 1959. They were sent back to Alco for rebuilding with 251B engines, and received RS-11 hoods. This rebuilding took place sometime in 1960. The RS-36 wasn't even in production until February, 1962, so how could these units have RS-36 hoods?
The 1613 and 1624 are well documented in Eric Hirsimaki's book, The Alco Line, with photos both before and after the rebuilding; the before photos are of the wreck, in fact. A wreck repair hardly qualifies as a feasibility study.