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back in the day no one measured the radiation. they sizzed it off the old boiler because bigger is always better.

One thing to consider if switching to atmospheric instead of power burners is the smoke pie size. This can be problematic in some situations. Typically the atmospheric boiler vents are approximately one third larger than the power burner boilers.

I'm almost certain this was designed as a Vacuum system, where a vacuum pump would draw the condensate up from those basement convectors and would also help pull air from the entire system. The receiver/pump with the box sitting on top is likely some sort of vacuum pump. @Pumpguy is our resident vacuum expert, hopefully he will see this.

1. the steam in the returns will kill the pumps in the condensate/feed tanks. Not sure why there are both. Not sure whoever put the last boiler in knew what they were doing either but pumped returns is a little above my head.
2. I am not sure how the basement is set up. Is there a pipe below the floor for condensate and a pipe up to the return for vent or just a pipe up? If there is just a pipe up it might need the 5psig to lift the condensate. If it isn't lifting condensate then the pressure probably can be much lower.
3. If it is short cycling on pressure either the system isn't consuming the steam that is produced (likely because the air can't get out because your returns are full of steam) or the burner should be modulating or dropping to low fire.
4. Figure out what is wrong and what needs to be fixed before adding insulation. Insulation will just make figuring out how it is piped and is supposed to work and making any changes harder.