400's are they any good?

If you go through all the money and trouble with machining for cross bolt mains, girdle, etc, economically you'll be at the same price for buying a new aftermarket block. Initially I was going to go the route you are talking about, but after I added it all up, I bagged the idea and ended up buying a brand new 6.1 Hemi for $4300.

Buy the book 'How to Build High Performance Big Block Mopars' by Andy Finkbeiner and you will learn a lot about these engines. The book is very good.

I know there are other schools of thought out there, this is my perspective. Good luck in whatever you decide.