Its BAM's catalog, I search your SN and it comes up with what I posted, but if I look its the wrong serial number range. I need to make sure to double check. The Mercruiser site comes correct

Pressure would be good to know, but as you said starting fluid changed nothing. I thinking more in the spark area

Good news and awful news.
The good: after swapping the rotor and distributor cap from my other engine I was able to get STRB to start. Although I still suspect bad fuel and think it was more just keep trying to start it. and dumping out fuel.

the awful: I warmed the engine up to change the oil to winterize it, and it came out like a milkshake! Yummy! But also not good. very not good. Lots of water in the oil! Good Times. Break Out Another Thousand.

So, bad intake manifold? cracked block? cracked head? blown head gasket?

The engine also leaks oil, so I've been pondering just getting a 'new base engine' for $3,000 for a GM 350 from Mitchigan Motorz. anyone ever deal with them? No core and they can ship to my house (no loading dock).

I'd also replace anything that's hard to get to or looks bad while I'm in there. Bilge pumps, oil filter lines/fittings/drive coupler/starter/flywheel/etc. anything that's hard to get to or under/behind the engine.

Or should I investigate more into a 20 year old abused block? I have friends and enough know how to do the swap (I think)...
My mobile mechanic wants to take a look before I commit to a new base engine, but I'm not sure the point. the oil came out black last year, not milkshake, so it must have happened over the summer. I winterized it myself last year, and it's in a heated garage. so I don't suspect freeze damage. The engine was almost 1.5 quarts low on oil over the summer. Maybe that killed it

Several on iboats have used Michigan motorz with good results. The ones that did have an issue MM step right up and fixed the issue

For me, sounds like the motor needs to be pulled apart and while its apart, I would rebuild. So either rebuild or replace

I have the skills to swap parts to the new engine. I don't have the skills to rebuild one... Thanks!

Well there is only one way to find out. Dive into it and take lots of pictures. Worst case you realize it isnt worth saving and buy a new one.

Compress test:
3 of the 8 cylinders show below 30 PSI. 5/8 are about 140-160.
I am prepping to remove the engine. I've removed the water in, electrical, exhaust, fuel, etc.
Hope to borrow a friends crane to pull this thing out and start tearing down.

I should most likely replace the exhaust manifold and elbows on the 'good' engine as well.

Already planning both on what will be the new engine as well.

I have purchased a 350 MAG MPI crate engine from MM about 4 years ago and I couldn't have been happier with my experience. They made the delivery a breeze to my door (private residence) and as I installed the motor myself they also helped me out to get the paperwork complete for my Mercruiser warranty. I would with out a doubt use them again if I were to be in need of another new engine. Motor is still running strong to this day never had any issues with it.

Good to hear and thank you for the feedback! Mine should arrive this week, and I can start the install the following week.