As you can see the in 1979 Boler Manual
in the Document Center, my trailer and others from the same source were supplied with either Hydro-Flame or Suburban furnaces.
While my 1979 Vanguard-built Boler
B1700RGH came with a Suburban Dynatrail
NT-12MEC direct-vent fan-forced furnace, some other 17' Bolers came with no-fan furnaces (presumably Hydro-Flame models).
There are lots of manuals for furnaces in the Fiberglass RV Document Center, and many will have dimensions and installation information. The dimensions given in my Suburban manual:
furnace height: 9 13/16", required opening height: 11 1/8"
NT-12 series furnace width: 8 1/8", required opening width: 10 3/8"
NT-16 series furnace width: 11", required opening width: 13 1/4"
installation depth min: 23 5/16"
installation depth max: 29"
Where not specified, dimensions for my NT-12 series furnace and the higher-output NT-16 series are the same. Note: these are not the same dimensions as the NT-xxSE furnaces.
There are all sorts of other installation restrictions and requirements - I would really encourage a thorough read of the manual.
To sort out the Suburban Dynatrail models...
NT-xx xx is the input rate in thousands of BTU per hour (output is about 3/4 of the input)
MEC = pilot ignition
MEF = electronic spark ignition (they had this 1979 and Boler
didn't get it! Arggggh!)
MEP = pilot with piezo ignitor (even that would be better than mine!)