Question or perhaps survey is better term. Looking for input on breaking in rebuilt inboards. Just to keep things in perspective, and to assure you I’m not hitting the easy button here, I can honestly say my personal research has not resulted in much agreement between sources.

My platform is a new reman 496 HO/Whipple. (Please withhold the time bomb jokes if possible.) I’ll simply say it wasn’t a bolt on stock build, but I digress. I’ve also not seen much distinction when it comes to breaking in engines with the exception of inboard, outboard, or automotive.

Needless to say, the breaking in procedures I’ve read are all over the place. Some indicate the procedures are build specific, referencing different procedures depending on roller or flat tappet cam. Some have detailed progressions like 1500-2000 for x hours, 1500-3000 for x, 1500-4000 for x, and then WOT after for some what might be the whole season.

I could go on with more examples, but needless to say, seems more art than science.

One last tidbit I’m looking for. What is the magic number or numbers when it come to the recommended first and perhaps second oil changes? 1,5,10,20 hours, etc?

I'd try to simulate a programmed dyno break-in procedure. Get it in the water. Fire it and take to 1000-1500rpm. Set the timing close and check for leaks. Then get it out and moderately accelerate and decelerate between 1500 and 3k for 10-15 minutes. Cool it off and get timing set where it should be. Double check everything. Then make a full pull from idle to WOT and back it off. Idle for a minute or two and repeat several times.
Heres why: It takes a load to properly seat the rings. Running it no load to break in a cam costs hp. That's a fact. I've broken in quite a few engines on a dyno this way. After the initial break in program we ring the snot out of 'em. Never had a problem with engines broken in this way. We've also taken known combinations and broke the cam in the old fashioned way. Those engines never made the HP/Torque of those done with a break-in under load.
Others opinions and experience may very.

I'd run a premium oil filter and change oil and filter after a couple hours. Sooner won't hurt a thing. If it's built right and everything was assembled clean a couple hours won't hurt it.

Assuming it was tuned on a dyno, that is all it should need, but I would do what your engine builder says.
I would break it in like you drive it and change the initial oil at 10 hours and for SC'd engines, I'd do oil changes in the 25-35 hr range.

I change the oil after the first startup, then I do a day on the motor and change it again.

Agree with tbev.

And cut open filters to inspect.

The worst thing you can do while breaking in an engine is baby the engine or allow it to idle for extended periods, particularly when it’s hot. That will place almost no load on the rings and it will allow that hone pattern to be broken down without actually performing its job. You want moderate amounts of load and moderate amounts of RPM. Mineral based oil for initial break in is also a good idea, not as slick as synthetic and will help with the break in. Change oil when done.

Late to the thread but just want to add that unless you did the engines yourself you really need to break them in following whatever procedure the engine builder recommends.

I will agree with the others that if they are broken in on the dyno that should be all they need.

Except for breaking in a flat tappet cam, a few easy heat cycles (running the engine gently and get it to operating temp and then let cool completely overnight), then run it like you stole it.