There are two factors associated with your question.
First, the engines should have the correct props to ensure that the engines are correctly "loaded". While this is usually measured, or evaluated, at wide open throttle (WOT), this correct loading affects the engine at all points in the operating RPM range. There are lots of lengthy posts and references on how to check to see if you are correctly propped, so I will not elaborate on it here.
Second, once the engines are "propped" correctly, then operating them in a reasonable, mature, manner will contribute significantly to their longevity. This probably means a nice cruising RPM in the range of 4000-4500. With those big engines, and the correct props, you may find a comfortable cruising speed at 3800RPM, so just do what feels good. I doubt that you will run that big 360 at a constant RPM that is bad for those beasts.
For reference, we repowered with our twin F150s in Nov 2005 and have 885 hours on them. We routinely cruise at 4200-4500, and I have made long runs in smooth water at 5000RPM.
Brian
