The Merc 525's are closed cooled so there's some comfort there, the headers tail pipes would be a concern, proper flushing after use will play a big part in overall condition, depends on how long its been is the environment, just the salt air can do a lot of damage. what's the hours on the engines ?

I think the OP needs to specify 525SC's or 525EFI's. Both engines are being referenced in responses and they are two different animals with their own set of issues/needs.

Also, keep in mind that yanking powerplants out of a Formula 400SS is no small task.

Curious, why the immediate assumption on needed rebuild? What is "hard use" on a crate racing engine? Agree with the other comments regarding salt, leaking headers.