Just got my Birchfield boilers from Multiscale Digital and they look great.
I have attached to photos of three boilers staged on a flatcar to simulate the load shown in the Birchfield Boiler photo that started this thread. I used two HO and one N scale boiler and I think that they will look great. And yes, the height of the car with the tallest boiler is within the NMRA standards with a little room to spare.
Now, can someone suggest materials and methods for tie downs?