I would tend to agree. Exhaust manifolds and/or a surge without the proper back flow prevention flappers, may have caused this damage.
On that note, I'd sooner see you retro-fit a 290 exhaust flapper onto the 280 exhaust outlet, than to install a new bellows flapper.
There is a hump on the 280 outlet that needs to be ground away, but it can be done.
Been there/done that many times.
I believe that the bellows flapper is too restrictive for any V-8 engine. Plus you cannot keep an eye on it as easily.
http://bpi.ebasicpower.com/c/VOL8D/graphics/00000001/MAL9-72803.jpg
That's just my take, FWIW.
BTW, typically we can build a 5.7L more cost effectively than we can build the 5.0L.
For a few hundred dollars, you have pick up a 5.7L core.
Steer clear of light cylinder head castings...... you can search this and see which castings to avoid.
Whatever you do, I'd strongly encourage you to steer completely clear of the GM style full dished pistons.
Instead, go back together with a piston that gives this engine a quench or squish effect.
Again.... only a few dollars away from doing a more Marine correct build!
Also, I'd strongly encourage you to replace the PDS bearings while the engine is out.
The engine must be out in order to access the double bearing PDS, of which I'm certain that your flywheel cover is using.
For about $45 you can buy the two bearings and the two new seals and a tube of proper grease...
not green marine grease, BTW.
Takes me about 1 hour to do this job once the transmission is out of the way..... of which it shouldl be for service work and bellows replacement.
This will prevent you from the destruction seen bellow.... when/if the rear bearing fails.
BTW, if the mechanic removed the flywheel cover with the engine, you'll want to replace the inboard rubber ring.... (aka rubber cushion).
This cushion provides the water seal between the flywheel cover and transom shield.
(the old cushion will have taken a set by now, and may not re-seal)
