DML FAQ : Engine : Can I use LA valve covers on a Magnum engine?
I'm using LA valve covers on my 318. (Actually, they're Sean's valve covers; I bought them from him when he sold his Dak.) They work great; no leaks, no fitment problems whatsoever. Just use a good quality silicone in addition to the gaskets. (I think I used "The Right Stuff") I don't recall the thread size offhand, but you should be able to use the stock fasteners. I did pick up some "T" style hold down bolts at Carlisle last year, and they worked so it is probably a common thread. (Actually, the T bolts worked except for the left rear bolt, the master cylinder is in the way, so I had to use a normal bolt in that location.
There's a description of the installation at http://www.dakota-truck.net/upgrades/LAVALVECOVERS/