All i can say is that old school steel frame bikes have worked very well for us
At least 12" of spread between the posts at the seat post location is plenty depending on how much tilt you want the engine too sit...
I don't have the measurements of the inner triangle on my fat tire bike,,, but it was plenty enough...
If I recall its a 11" in-side measurement in the triangle,,, its an aluminum frame bike for slow speed operations...
If I was planning on rugged off off roading I'd add re-enforcement posts with clamps too beef it up...
The same concept as the first mountain bikes that were used on the Re-pack down hill mountain bike run...
They either welded or tig-welded the re-enforcement posts in place too strengthen up the main posts...
Just throwing out some ideas is all,,, like I mentioned,,, its hard too beat most quality steel frame bikes,,, for us that is...
Jt