I have a 1972 Crown wagon that I am finally ready to put some money into.
Finding parts is of course very difficult. I have noticed that '72 mark ii has the
same engine as mine (4m 2600cc). My question is, are all of the mechanical parts interchangeable? I am talking about engine parts and suspension and brakes, basically anything not dealing with the body.

Thank you so much!

Simple answer to your question is no. They are not the same.

They are very similar, but not everything interchanges between the two.

Engine parts are basically what you can interchange. thats about it.

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NOT STOCK!

I would highly recommend utilizing RockAuto.com for some research. If you click individual part numbers it brings up a cross-reference of what other vehicles the part is sold for. It has come in very handy!