by making more power, I will need to re-prop it, that I agree. The prop will ultimately determine the engine redline, which i think 5000-5500 is a good target. My thoughts are the added tq would be especially beneficial to a marine application because most its life is going to be in the 2500-3500 rpm range. If properly built, I don’t see why a marine 383 would not be just as reliable as 355.
I also would rather use a 6.0" rod for exactly your reason Mike, less pressure on the cylinder wall. One of the local machine shops recommended against it claiming poor reliability. (which I don’t exactly buy into).