It may be that I am just all thumbs when it comes to this stuff, but ...
I removed the boiler (fairly easy) to paint my K-27. I painted it. Then I tried to put it back together! What a nightmare. Like I say, it might just be me, but I could not get the Smokebox to clip over the tab on the frame (Smokebox Support). No matter what I tried it would not go, and i tried everythig. I finally resorted to grinding the lip off the Smokebox Support so the boiler shell just goes on straight down onto the frame. Then it worked fine. I have had trouble before getting Bachmann locos apart because the exploded drawings (which are great for parts identification) don't tell you how to take them apart. At least the K-27 instructions told me how to remove the boiler shell. All-in-all, the K-27 is a really nice loco, but I sure sweated over that boiler shell reassembly! ???
I sincerely hope you didn't grind it off.
I am not sure I would trust the standoff under the sand dome to do the work.
I've been into 6 of them now, and it it easier if you (A) remove the smokebox front, and (B) bevel the bottom edge of the lip on the chassis and bevel the back top of the lip under the smokebox.
I had a problem with mine also. My solution was to raise the cab an inch or 2 and slip the smoke box over the tab first. then work the cab down to its final position. I have to admit it had me worried there for a few min.