While the GN and PRR were by far the largest users of Belpaire fireboxes and also used them for a much longer time, it is incorrect to say that ONLY these American railroads had them. There were also these (and likely others):
Saint Lawrence and Adirondack
Illinois Central
Lake Shore and Michigan Southern
Flint and Pere Marquette
Duluth, Mississippi River and Northern
Louisville and Nashville
Oregon Railroad and Navigation
Chicago, Burlington and Quincy
Wisconsin Central
Buffalo, Rochester & Pittsburgh
Indiana and Illinois Southern
Union Pacific
Monon
Peoria and Pekin Union
Louisville New Albany and Corydon
Toledo, Peoria and Western
Jacksonville Southeastern
Northern Pacific (Class Z 2-6-6-2)
I have added Belpaire fireboxes to two Mantua/Tyco steam engines: a 4-6-2 and an 0-6-0. For both, I built a dam around the top half of the existing firebox and poured in epoxy. I then did a lot of filing and filling. It worked quite nicely. I did add boiler banding and various appliances.
Ed