From the description you give, I would be tempted to leave well alone. The rocker box will not crack due to differential expansion because the holes around the valve guides are bigger than the external diameter of the guides. If you take the guides out, you will cause a degree of wear in the holes in the head. While it will take several guide removals and replacements to cause a problem with loose guides, the less this is done the better. If you do remove the guides, you will find that you need a variety of gaskets on reassembly, all of which need to be exactly the same thickness. I had difficulty finding a full set for my current 4G when I rebuilt it. I had to replace the guides as the old ones were very well worn. If you talk to Bruce Longman he will tell you that it is well nigh impossible to achieve a seal between the rocker box and the head. In his experience 500 miles is a good mileage before the leaks start. I have managed that and more with both my 4Gs now but slight leakage is becoming less slight, I fear. It is significant that the Mk I head is specifically designed to overcome this leakage issue.
You won't lose much by leaving well alone - perhaps a set of head gaskets if the worst comes to the worst. Diving in, without availability of replacement cast iron guides, is a big risk in my book. But Drags had replacement guides until recently so give them a ring and see what the prospect of availability is if you do decide to replace. I would not fit bronze guides due to their greater expansion than cast iron; which seems to me a recipe for trouble in one way or another.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.