That could be difficult. Athearn, for some reason, doesn't make their boxes MT-compatible. Most Atlas body-mount, with the Accumates, are, and 1015s drop right in. But not Athearn
Depending on why you want to be rid of the McHenry's, Accumates might be an option. You could replace those on Atlas cars with MT, then use the Accumates on the Athearn cars.