prepping for paint

I have a 74 Dart. The front clip was trash. Got a clip from a 73 Duster. I've pulled the 318 and 904 (I'll be putting in a magnum 5.9 w/ a727)

There are years and up to 3 inches (no joke) in some places, of dirt and grime. I'm thinking of renting a pressure washer to remove most of the build up. But I have some questions. Should I use a hot or cold pressure washer and how many PSI's will be enough?

Or is there a better way to go about cleaning up this mess?

Hillbillybuddha