340 X head max spring tolerance?

The springs aren't the only thing that affects how much lift a head can tolerate. The biggest factor is retainer to guide interference. To run a .600 lift cam you'll need to have the valve guides cut down and probably have the spring base cut for more spring installed height.

Your gonna spend some big money porting them and getting them machined to accept .600 lift. Plus the cost of all the parts. You'd be better off buying a new set of RHS heads (or Eddy's) and sell your X heads to someone that needs X heads, IMO.