woodbowl,
I do not have any first hand info on the 35 hp B&S, nor any on a mill engine set up.
I do know from what little I have seen, that the mill stress on an engine is going to be more similar to that of a motorcyle than a lawnmower.
What I have found in a myriad of motorcycle engine/applications is the the balance of bore/stroke ratio does tend to have a direct influence on shaft/bearing life. Oversquare (large bore/short stroke), though good for rpm transfer capability, has shown to create a notable shorter shaft/bearing life when run hard.
Have been able to compensate for that some with a wider variety of gear ratios and oil coolers.
Hmmm...jp, I see you have been digging around a little.

In honesty, that engine you have setting there....well... there is part of me that really does itch to hear it sing. It should make one heck of a mill engine.