I've been contemplating going to bigger valves in my GX head, the next time I rebuild my A15. I'm told that It's safe to go to 42mm inlets, and 35m exhausts. (Same as L18)
I'd just use A12 inlet valves for the exhaust, and do the cut down L18 thing with lash caps for the inlets. (unless I can find a valve the right length from another engine)
something which has sprung to mind, (I'm hoping someone who is running these valves can advise me) is when running valves this big, is it necessary to machine releifs into the bore walls to allow clearance between the valve and the bore wall/ blockdeck?
The A12's have this, as do the A14/15's to a much lesser extent. (My a15 is .040 over, so these reliefs are pretty much gone) I realise that the valve doesn't protrude far below the deck.
I heard a story from my mates dad who is a mechanic. A customer of his tried to soup up a triumph. He got some flash harry mechanics to fit a bigger cam. When the valves hit the block deck, it kept snapping rocker arms. They kept on fitting new ones. My mates dad got the "pleasurable" task of repairing all the damage, after they finally came to the realiseation that something wasn't right. So keeping with that theme, I'm a firm believer in thinking through these sorts of things long before ever picking up a spanner. The other thing I was concerned about, is I want to have the block O ringed for the next build. But I'm unsure about how valve releifs in the cylinder wall might reduce the room for this to be done.
so what has everyone else done in these circumstances?

I'd have thought valve of those sizes would be touching each other (on standard centres)? Not sure about using Inlet valves for Exhaust - aren't exhausts different material to withstand the heat? I cut down L series exhaust valve diametre (and of course length) to fit.

If you use the L series one and cut them down do they need to be heat treated again or do you just throw them in????

I've read that you just fit lash caps to the end, to prevent it mushrooming over where it's been shortened.
Someone told me that L18 valves fit. I assumed that they must use the same stem size. I'm sure as long as they're reasonably close the guide can be opened up a bit.

Im running 40mm inlets and 33mm exhaust valves in the reg cook head i have. Its his race head from the mid 80's.
At 40 and 33 i would honestly say that i do not think there is any possibility of fitting larger valves. I think the head is originally a late a13 head however. The valves are so close together, that if they hadnt been run previously i would think they would touch.
I've been told by marsh that they are valves from a crossflow ford ??

No sh1t. I just sold a crossflow ford engine this morning.
A bloke from Oamaru in the DOCNZ said he was running 180B valves in his A series, so I was going off what he said.