If you want to do a head swap and go to hydraulic cam ect. The guy to call is Bob Madara of Marine Kinetics. Forget what you know from cars, and listen to what he has to say. Cars and boats don't mix. If one can grind you a cam, he can, and advice you to what heads to use for your intended purpose.

The big problem you have, is you need to identify what you have, unfortunately there is no "easy" way to do that....the fact they are laying over right at 4k makes me think valve train sounds like you aren't getting enough air.

You are correct you need to be conservative with the duration on marine engines for reversion, and there is only one guy to call for that -- Bob Madera from Marine Kinetics.

I don't know why he would say 86's are weak though, I had an 87' Liberator 350/260 and I hung out with a couple guys that had 86' and another with an 84' and they were all damn near identical as far as performance. We all turned 21" props right around 4800rpm's

If it were me, I'd pop off the front cover and try to identify the cams, or pop a valve cover and check the lift (you should be able to pull a # of the end of the cam and identify it). I mean, does it look like they have ever been apart? Obviously don't do this until you check the timing etc. This is where it can get REALLY expensive if you are not careful....Without knowing what you have it's tough to decide what to do -- your engine guy is right, bolting all those high dollar parts on are worthless without the right cam, especially if someone put towing/RV/truck cams in.....