Hey Toybrota,
Well… it's great that you got your engine to fire right up and it sounded really good. 
Leaks at 1st time start ups are not uncommon. I’ve had coolant leaks almost every time I have fired a fully rebuilt or partially rebuilt engine.
I am very careful about trusting what any auto parts store says, they’re in the “sell-you-everything” business. 
Regarding the block heater, when I looked at the photo I could not see how it would have enough contact with the block freeze plug bore to sufficiently seal. But I have never used a block heater. As you know, freeze plugs are pounded in under a very tight tolerance. The amount of pressure (pounds per square inch) on those plugs is about 30 PSI if your radiator cap is 13 lb. cap! Risking a coolant leak that size would be crazy. I would consider a different block heater. 
Pressure testing a coolant system when an engine is cold and pressure testing when it’s at normal running temperature are two different things. 
My guess is the reason it didn’t restart is probably mixture (air/fuel) issue and nothing mechanical. 
If engine ran for about 60 seconds, that is plenty of time to get the oil up into the rockers, cam, through all the bearings and cylinder walls. 
Obviously you got another 15 minutes at RPMs to complete the camshaft break-in.
Gnarls. 
The engine ran great! Everything is Definitely oiled up well.
I am doing a different block heater, I'm returning the one I got in favor of a magnetic electrical heater that goes on your oil pan. For the mean time, I will be using a rubber freeze plug in the hole I pulled the heater out of.
Coolant leaks are common on first start up, you are right. That's why checking the system with a tester is important. If I can pressurize the system to 13 PSI (rad cap threshold) then I don't have any leaks.
Before I start it up I am going to verify timing again (pulling valve cover and making sure it's all good)
I'd like to note as well, this thing didn't have a single leak prior to the little blow out, everything else is sealed properly!
I plan to get the break-in done today after I fix the freeze-plug. One thing I noticed is the oil pressure, over 60psi at hot idle. I expect about as much with the new pump. The pressure is constant too, as fluctuating numbers can attribute to blocked oil passages... Not for me.
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