I noticed the 2.5 diameters also. Not sure where that came from. If it's about engagement of threads, 4 treads is as strong as the bolt.
Nuts are already designed to give the 4 threads to make them as strong as bolt. You don't have to worry about that.
If it's a tapped hole in steel, it's still 4 threads or more.
Aluminum: helicoils.
1.5D engagement on a 1/4-20 is 7.5 threads. Engineers and Designers don't use rules of thumb. They use math and testing.
Regarding the clamping force of UNC vs UNF. When properly torqued, which is very iffy, a UNF only gives 13% more force. This is not a factor in old truck design. The UNF was used for other reasons. Hinge plates don't squeeze at torque levels we use.
A hard joint is what we are all used to. A soft joint is when a gasket or spring or rubber is involved, as an example.
