All they are after is the "cheats" and looking at the compliance issues sometimes raised with crate engines being late model blocks which the require to match emissions.
It is like all things 'if you tell the truth you never get screwed'.
As for pulling out engines etc, 'the may be dumb, but they are not stupid' in 40 years of importing I have never had a major hassle and actually the biggest was when some genuine Mexican left his Big Mac with a bite out of it, Fries and Shake sitting on a carton in the pan, and that made them panic.
Just list what you have show how you bought it and give them the photos that show a thousand stories. Bucking the system causes lots of grief, using it to your advantage is the only way/