What would you do?

Did you end up with 2 hole or 4 hole injectors. Where did you get them, what is the part number, info for the next guy that has this problem would be great.

poecs13 wrote: I was able to blow through the line without too much resistance, but maybe the interior has deteriorated so there isn't adequate flow back to the tank? Does this seem plausible as a cause and viable solution.

No
if you can blow though it fuel can flow through it. I’m not very familiar with the Chevy system but I would assume there has to be a pressure regulator on the return line, that’s where I would start.

What would you do... for a Klondike Bar?

Pull your fuel rails, leave the injectors and fuel lines attached. Put a thick rag topped with clean paper towels beneath each injector tip. Turn on the key to prime the fuel system, then turn the key off. Come back after an hour and see which injector is leaking down - you'll see gas soaked into the paper towel below the bad injector.

I had this problem on a BMW 330i. Car ran perfectly but was miserable to get started. Bad #4 injector caused the problem.

Sounds to me like the injector issue was never fixed. 4 hole or 2 hole does not matter but what does matter is the gallons per hour flow and a quality build.
Many GM drivers tune for more horsepower with the LS engine blocks.   A common upgrade is to replace standard injectors with new injectors that have a higher gallon per hour flow rate to keep up with the other performance mods they make.
The only source for fuel in the intake is your injectors, if you are flooding they are not holding the fuel.   
Opening the throttle wide open while starting in modern fuel injected engines will shut off your fuel flow, this is an automatic built in process to help clear a flooded engine.
The fact you have to do this to start does again point to injectors as your problem.
I would not change any other parts till you fix this issue.
Yours failed at a very young age, not sure why but maybe the previous owner ran a bad injector cleaner through your system and damaged them.   I would replace all 8 before you do more damage to your brand new engine.   
GM does not make fuel injectors, they are probably OEM Bosch or Denso brand sold by GM.   Take a close up picture of your injector, the part number is usually printed right on it.
Many companies sell OEM type injectors that will swap in with zero issue.   I would do some reading on the Camaro and Corvette forums to see what brand those guys are happy with, I would not think twice about Marine, not important for this part.
If your OEM injectors are 24 lb injectors I would stay close to that number for your boat, if you find 26 or 28 lb injectors for a good price use them.   A small change will not affect you but stay with the same if possible. Do some reading or ask questions and make sure the new injectors are E85 compatible, that would allow more fuel choices in the future.
How does the new engine run?   I hope it makes you happy when it is running.

Ken,

Thanks for the manual! I hadn't come across that one yet, but it is super helpful.

However, the manual states that the combo MAP/IAT sensor only exists on the 5.0/5.7 motors. On the 6.0, it's only a MAP sensor. Is the MAP sensor still the potential culprit in this case?

Ken,

Thanks for checking in, I've been meaning to get back on here to write an update. This was the kick I needed.

I wish I had better news, but I'm still dealing with the same hot start issue that I chronicled here before the hydrolock and engine rebuild. Hard to believe that it's been almost two years and I still haven't solved the problem...

It takes one crank to start the engine cold, no problem. After the engine is warm, if it sits for more than ten minutes after shut off, it is flooded and I have open the throttle wide open to clear it to restart.

On the post referenced above, Jody suggested replacing the air temp sensor in the flame arrestor, but all I have near my flame arrestor sensor-wise is a Throttle Position Sensor and an Idle Air Control Valve. Unless one of them also has a temperature sensor?

I am open to any and all suggestions on this one. My wife is ready to trade the boat in. She doesn't want to go another summer with a non-functioning boat and is (rightfully) fearful of repeating the hydrolock episode from last spring if we continue to run as is.

I would want them to match. I still doubt the number of holes needs to match as long as the volume/pressure matches...

I’m gonna use some JB Weld there Paul- it’s got weld right them there in its name

Grab a set of either GM label or Delphi injectors. Can be found easily online. Get a set of 8 so they all match.

poecs13 wrote: Right, but apparently they come in a two or four hole variety, does that make a difference? The engine currently has the four hole version.

Does on a Ford,the high hp ones have the 4 hole. Just to be safe I would not change

Right, but apparently they come in a two or four hole variety, does that make a difference? The engine currently has the four hole version.

I figured the injector shop would be able to give a recommendation, but I said the word "marine" and their eyes glazed over.

poecs13 wrote: From the shop that worked the injectors today: "One of them was stuck wide open. The first set of four took four cleanings before they flowed correctly. The second set took three cleanings to flow correctly. They all seem to be flowing within two to three cc's of each other.

Your injector shop should be able to advise you on their size and alternatives

This is the PCM one that my boat needs.

What I'm looking for is an automotive equivalent from ACDelco or the like for cheaper than PCM's $200 a piece.