I don't know about the MTH product.
However, the prototype J Class had a wheelbase including tender of 95 feet 4 1/2 inches.
The Atlas TT is think is 24 inches long which is 96 feet in O scale. So if the model, is accurate to prototpye, it would just fit. The flanges might not.
However, the MTH version may not be that accurate to scale. At the very least, the tender is probably not couple so close to the engine so it can go around the curves on your layout. That would make it longer. On the other hand, mfg often shorten models because they do not have good information or for some other reason. If MTH shortened the loco or the tender, then it might fit.
You probably need to ask someone who has one, what the wheelbase is.
On a very tight fit like that, it would be very hard to get the loco stopped precisely enough to clear the ends of the TT tracks. I think for a loco like that you might want to consider a 30 inch long TT which is a 120 feet long. The prototype J is 109 feet and some inches long over couplers. On a 120 ft table, you could see the ends of the TT track for alignment.
Many many TTs in the US were 100 feet long. Many RRs had a place or two where they could turn big stuff, but not on every TT.
I got these dimensions for the prototype J from an article I looked up on the internet The article included a reproduction of a blueprint drawing.
