Dawgie?
not sure what your engineering background is but most issues with auto conversions are engines running at or near manufactured HP and torque ratings, and no transmission to act as a buffer, not to mention pulse fatigue and shaft alignment issues. As far as constant power stresses go at prolonged cruise settings, again mist auto conversions operate near 100 percent of max rated power. Marine engines have far more stress than aircraft engines and do fine.
Check out
www.corsairpower.com. They have a V8 engine for Cessna's with a lot of info on the topic.