These days with unleaded fuel unless you are using 21/4N stainless steel valves with unplated stems (like me) I would use the stock cast iron guides.
The other guides you are thinking of are manganese bronze, not magnesium (which would last about 5 seconds).
these are the ones I use in my 1360 for reason above.
I never use valve stem seals on the exhausts, there is no vacuum there (unlike the inlets) and as Vizard says they need some lube for heat transfer. Particularly bronze ones- they can seize with no lube.
I know that BL/Rover fitted seals to all 8 valves from around 1980 onwards, but they are pretty ineffective once they go hard. It doesn't take long at all....
