Derailed said:
… as long as the fireman’s walkway plate (not sure what its really called) between the engine and the tender is removed. …
Uh oh, the suits won’t like that!
§ 230.81 Cab aprons.
(a) General provisions. Cab aprons shall be of proper length and width to ensure safety. Cab aprons shall be securely hinged, maintained in a safe and suitable condition for service, and roughened, or other provision made, to afford secure footing.
(b) Width of apron. The cab apron shall be of a sufficient width to prevent, when the drawbar is disconnected and the safety chains or the safety bars are taut, the apron from dropping between the steam locomotive and tender.
https://www.law.cornell.edu/cfr/text/49/230.81
As it happens, several rounds “try it and see what happens” shaping - AFTER installation! - were recently required on an apron I made for a Bachmann HO scale 0-6-0 for exactly the problem you are having; in my case it turned out the apron was impacting the inside corners of the water legs as the locomotive’s long rear overhang swung off and back on to the track centerline.
And here’s after just enough filing to reshape the apron enough to solve the problem, still a realy close call but the problem has stopped,