installing different springs and modified retainers wont give you more clearance, but if you install them at a different height, thus increasing the lb of the spring, you could get more clearance. I dont know a whole lot about cams and rocker combinations, but from what I've read, doing higher ratio rockers with a cam doesn't help very much. for instance, overkill has a custom grind cam that he designed for use with higher ratio rockers and he advertises an additional 10 horsepower. not much more horsepower for the 400 dollars or so for a new set of higher ratio rockers.
as far as I know, milling the valve guides is all that needs to be done for valvetrain clearance issues.
