While I like the sound of a V8, the 304 really isn't much, if any, improvement on the 258. A 304 will also be harder to find than a 258 or any of the other, larger AMC V8's (360, 401). Although there are the short deck 343/390's around, probably best to stay away from those from a parts availability standpoint. If you want an AMC V8, the 360's are by far the most common and will bolt-in just like the 304.
That said, I'm 100% with Harley and if doing any engine swaps in CJ's, the ubiquitous small block Chevy is my only choice. You can build them just about in any configuration you can think of in at least a dozen different displacements with all factory parts. And you can't find any cheaper parts. That and they just plain work. Way more power for the buck than any of the AMC's and, in my experience, they get way better mileage.
Best application for AMC V8's? Do you have a boat and need a mooring device?
1947 CJ 2A, 1949 CJ 3A, 1955 CJ 3B, 1968 CJ 5, 2012 JKUR