What to look for in U-pull Magnum?

I'm seriously considering swapping a 318 Magnum into my '68 frankenstien Dart project.

There's plenty of them in the local U-pull yard, and I can pull the pan and heads right there for a look at the cylinder walls, bearings, etc.

Other than the basics, are there any problem areas to be on the lookout for with the Magnum? I remember reading something about heads cracking between the valve seats. Is this visable by looking? I'm assuming it would also make for a suspiciously sparkling clean piston and combustion chamber too.

Anything else?

Thanks