The difference between con rod nut torque in the pushrod vs the XK engine might be related to a number of factors.
The pushrod bolt is 3/8-22 BSF where the XK bolt is 3/8-24 ANF thread, so there is a small difference in stress area at the root of the threads.
The composition of the steels used and thus the proof loads are unknown and may be different. Proof load is usually about 2 percent below the yield load, the point at which the bolt stretches in the plastic range, i.e. permanent deformation.
The pushrod torque was specified by the engineers at Standard Motor Co. where with the XK engine it was Jaguar engineers, who may have had a different philosophy on the subject.
The pushrod engine is intended to run at under 4500 rpm, where the XK engine peak is 5200 rpm.
Torque is actually a secondary method of achieving the primary goal, which is a certain preload. Some engine builders measure bolt stretch.
The greatest inertia load on the bolt occurs on the intake stroke, and this plus the preload is the maximum loading condition on the bolt. The maximum load times the number of cycles is the fatigue loading. If the fatigue loading always stays under some value known as the endurance limit, usually 35 to 60 percent of the ultimate tensile strength, the bolt can last indefinitely.
However, running with worn out knocking bearing shells will increase the inertia loading and reduces the fatigue strength. This is the reason the factory recommended replacing them when replacing worn bearing shells.
reference Mechanical Engineering Design by Shigley and Mitchell, chapter 7 Design for Fatigue Strength.
Search the subject on the internet and you will find many studies, recommendations, and opinions, some of which are colored by catastrophic experiences and some by sales pitches. The bolt sellers are all in favor of replacing them as often as possible, as are race engine builders working on somebody else’s ticket and having to guarantee their work. I found one drag racer that likes to change them after every weekend at the strip. But by contrast there are non-racers running low stress engines that feel that with new bearing shells and careful measurement and assembly, they are not worried about con rod bolt failure, and would rather spend their money elsewhere.
Notice I’m not giving my opinion here, just reporting engineering facts for which I have great interest, and the existence of other opinions for which I have less interest.