We went through this before... A K28 has a very low cab. the cab floor is the same level as the firing deck. All the other K's have a step up from the deck to the floor where the seats are.
K-27's have the shortest step, and the air pump definitely intrudes into the fireman's view of the track. If the cab on a K27 was lowered and the fireman's seat was the same level as the firing deck, visibility would certainly be a problem.
K28's already have rather limited visibilty ahead because of the high running boards over the air reservoirs. If you hung the air pump on the side of the boiler it would have stuck up through the running board at least as much as it does on a K36, if not higher. The view ahead would be certainly compromised.
Oahu Ry's engines used a pair of 9-1/2" single lung pumps which are much shorter than a cross compound pump.
The solution to the problem would have been to raise the cab a foot or so.
That's my take on it...