So bent a rod at startup in one of my 17 year old 350 Mag MPIs. Not many hours on the motor but looks like age just caught up to her being raw water and running in salt, although avid flusher as the boat is trailer kept.

Being their age, I've decided to just go ahead and replace both engines just for piece of mind. Sat out all last season in preparation.. Looking around at options, Ideally would like brand new 350 long block, no reman's, know of several places that have brand new Merc replacements. This way I can reuse most of the parts off my 350 Mag MPI, fuel rails, throttle body, MEFI3. Buddy put a single GM 502 HO crate motor in his setup. Got me looking at these myself now, the HT383 seems like it would be a perfect fit. 320HP & 444lbft at 3000 RPM. Just don't know what, if anything, I could reuse from my 350s. I'm not looking to get 10 more MPHs or anything like that, also not trying to break the bank on these builds either. That turn key reliability that I've had for the past 10 years is what I'm looking for. Just wondering if anyone has any experience with these GM Performance Crate motors in marine setups.

GM Performance Crate Motors

It sounds like you need to stick with stock so that your MPI setup will work. Until 2005, the camshaft was the same in all 5.7's, so as long as you get a marine longblock from somewhere reputable, you'll be fine. 2005+ MPI had a very minor difference in cam.

If you want a new marine 5.7 with the correct cam, I would talk to Marine Power. The GM Performance is going to have a automotive profile cam that you and your marine MPI aren't going to like.

I am not an expert on this, but the subject of automotive engines being used in marine applications comes up often.
I think about it this way... a marine engine has no transmission and goes from idle to pinned for may be 2 minutes, maybe 5 minutes, maybe near redline on a lake, river, ocean, whatever for a very long time.
When was the last time a Vette 6.2 was pinned for 15 -20 minutes going from a start of a journey to another, continuous. near redline rpms for extended periods of time.
The engines have the same basics but not the same development.

There are also a number of builders that have adapted these engines to marine use but that is not plug and play.

a street car engine may live with 7/10 th of a thousand inches of clearance between piston and cylinder walls. a race track car engine needs more like minimum 1.5 to 2 thousand of an inches. then , a marine engine, because of the load and heat created, I'm not sure of the mesures but do recall it needs more like 3-4 thousands of an inche... so it is the same with every moving parts. my guess is some to most component of a car engine might be good enough in a boat, but a couple things needs to be adresses, like clearances, like better valves, like cam profile, to get reliability. and best performances... so whatever your choose, get some professionnal advice so you don't have to spend twice when your thought you made a good deal!!!

but if ever you choose a crate, i can sell you one !!!

get hold of frank at FU marine in Elkton, he has built a couple small blocks that run great and have efi.. he has a 383 in his 21 Apache