Kadee number 37 is a bit tricky to play with. Number 5 or the whisker type (147) are way better. You might have to do a bit of filing, but they should fit.
By the way, adding all-wheel power pickup on the tender will improve the performance drastically.
Simon
A Kadee #5 should fit just fine, as that would have been one of the first Kadee couplers with magnetic uncoupling in the time that loco was first popular. A 2-56 screw was the go-to at that time.I prefer flat-head screws to attach the coupler, as they're fairly easy to countersink into most Kadee draught gear boxes: use a drill bit just slightly larger than the diameter of the screw's head (#19 or #20 or 11/64" if you prefer SAE) to make a slight depression in the cover (you can do it by hand), then follow-up with a smaller drill bit. This won't completely countersink the screwhead, but it will at least make it much less noticeable than a round-headed screw.
This is Bowser's version of that slope-backed tender, with a Kadee coupler, also screw-mounted...
...although the box looks narrower than the original one used for the #5 coupler...
Wayne
https://www.kadee.com/hocc_mantua.htm
Any suggestions for specific Kadee coupler/box/etc to use for this Mantua slopback tender? I'd prefer to use a whisker coupler. Any filing required? Would screws be 2-56 or 0-80? Not sure which coupler will fit this. Thanks in dvance!