Stephen, if you have an aluminium head and if the valves are bronze alloy, unless you have good workshop facilities and experience of doing these, I strongly suggest you take it to someone who has done this sucessfully before. They are a problem to get out and the bores in the guides can be damaged drifting the new ones back in. You will need a hot head and/or very cold guides to do this without problems. I kid you not.
Send the head to SRM and save yourself the hassle, they will check your head bores once the quides are out and fit ones with the correct interference in the head to suit each valve position.
Good luck anyhow.
Alan