With that year engine ,it should be mass air,not speed density ,so you could install a larger cam,larger injectors.............but it may require some computer tune ups.......

I really don`t know all the ins and out for FI ......A stock fuel injection system can support close to 500 hp due to the pressure it runs at,you may need things like a aftermarkett fuel rail etc in some cases.

A good place for info that is 460 related is 460ford.com ,those guys are very exsperianced in these engines.....do some reading on thier website,most of what you are asking has already been covered there....use the search on that site for answers.

I wish I knew more to help you better on the FI stuff.

I can tell you a good rv cam and some headers would do wonders for your application,might not get 400 hp ,but the torque would go up and the headers would help it breath better.
Talk to one of the cam manufactures they will be able to tell you what cams are compatable with your application and needs.