For those who are visual 1/29th scale equipment is alas too big to go behind 1/32 scale equipment, I know because I tried. When I got my MTH GG1, 14 years ago, I had a buddy who lent me his Aristo Craft Crescent limited cars. They were a cm too wide and almost an inch too high. It doesn't show too much behind a Big Boy because these engines had huge tenders. But behind a Gee it really shows. (Remember that most railroads built east of Chicago had a much smaller loading gauge than western roads). I am sad that this is so, because I would just love to put the USA trains Congo cars set behind my Gee...
Now to answer your interesting question Gauge one bloke, it can be done although puttying between the flutes of the corugated sides would be difficult if not impossible at the splice. However, the trucks have been shortened also. I know this because, at first, I thought about doing this myself, but on smooth side cars (PRR Tuscan). Luckily I was able to get some of David Leeches cars which are aluminum, full length and from authentic PRR diagrams. That is not possible any more except on the used market. The Accucraft cars would be your best bet and would probably be easier to come by. However they still haven't produced stailesss teel fluted cars.
By the way another really troubling feature of the MTH cars is the fact that their windows are not flush with the sides as they should be this is very visible.