There is no correlation of P8 to 737 - probably due to significant structural and system differences. P8 wings might be similar to a 737-800 but perhaps it has hard points?

Atlas is the same but there is only one civil registered...I believe?

King Air 350 is similar to Shadow but, again, Shadow has has some large mods to get where it is today, though not as large as the P8. I investigated doing similar DLR and Optical/IR mods for a civil anti-pirate project. (but without the Snooping gear I think?)

The MAA/EDA are nowhere close to the CAA/EASA which are supposed to be independent of other authorities. The MAA is definitely not independent of any MOD influences and, as if to prove it, has 'developed' in some places the antithesis of CAA regulations and quietly ignores others of its own. Do not confuse the two regulations, I have worked with both, separately, for some years.

I'm showing my age when I say that a former Maintenance Manager I knew (RIP) for Bristow Helicopters applied for and got his first Section L AML with a Whirlwind Type Rating because he was "Q"d on Whirlwinds in the RAF! (different times, I know). The CAA and EASA clearly legislate the training required now for their Part 66 licences and, to be honest, there are only two differences between them which are: a. the need to Log their specified aircraft State registrations for your experience logs and b. the covers of the regulations used. If you base your Any Part 66 commonality would probably be in the Maths and Theory of Flight exams - and there aren't too many of those. My experience of Licensing ceased almost a year ago so I may be a little out of date.