you probably need to go to 17p version of same prop,
you probably wont go much faster but your engines will last a hell of a lot longer, right now it sounds like you are lugging the heck out of them.
Another thing is what are your drive and tab trim settings ?
If you are not trimming boat all the way out you will see some slow numbers as well especially as underpowered as it is..

As a rookie I'm not well versed on what trim tab and drive trims I should run, and would appreciate any advice. By the above I assume you mean I should trim the drives out as much as possible (before cavitating). What should I do with the tabs? Do I want the boat riding bow high?

I take the advice of the poster with the same boat and 365 big blocks, however 58-60 that you're getting is 50% more than me at 40 mph - would that mean my 350 magnums are making only 240hp, which is probably right given they're only revving to 3300/3400 rpms!

I'd still have to imagine if I can get them to 5000 odd where at least they'll have a chance of making rated power I can go faster. Maybe 45-50 is as good as I can expect.

you want as much boat out of the water as possible.
Typically for max speed you trim out drives until your mph stops
increasing and tabs should be fully up so as not to creat any drag at all..

The 350 Magnum in my ski boat is only 265 hp.

Agreed