Looking at Barts SC head and Peters gx head for sale
they both have 42mm inlets and 3Xmm valves for exhaust.

However for turbo boost which would be best?

42/30
41/31
40/32
39/33
38/34

Supercharged

42/30
41/31
40/32
39/33
38/34

Do the 42mm valves make the between the seat area to narrow or subject to crack under high pressue?

Generally larger valves will be best for high RPM, turbosupercharging or blower supercharging.

Although you don't want extra narrow valve seats with high boost. Seat width determines how much transfer heat goes from valve to engine coolant.

I always thought larger on the exhaust.

The boost helps make up for smaller intakes by simply forcing the air in but the larger exhaust helps spool up a turbo quicker?

Also as DD says, boost will bring extra heat with it, especially if turbo'd so you need to take that into account with you seat thicknesses etc.

From what I've read bringing the exhaust valve closer to the inlet size is the go (since the loss of exhaust energy becomes more important). It's the exhaust valve that cops most of the heat so you want to keep the guide and seat area up. The inlet still gets cooled by the incoming charge.

Supercharger; the exhaust valve area is even less important since the waste energy increases. Problems with heat are probably about the same. You can always add water to keep the chamber temps down.

An "expert" reckoned my 42mm valves were a wank, and 39(?) was as big as it was work going as gains deminished above that. Maybe he's right? I bought my head with them already in. They had a few cracked seats was all. Few hundy bucks later it was good to go.

EDIT I think with a super'd engine total CFM is more critical than velocity. People often say that PEAK CFM doesn't tell the whole story, and good velocity prevents an engine being too doughy down low. The ZERO lag factor cancels that out IMO. (Just based on my own random rational, and not on actual facts)

I heard the same as A14forced: S/C motors dont need larger valves like N/A motors cause it's forced in anyway. And Larger exhaust on a turbo

True, forced induction does force air through restrictive valves. But it can make even more power if it's not restricted.

I read the exhaust side is equally important to expel the gasses out on the supercharger as they are necessary to spool up a turbo?
However a suckthrough turbo could be different to drawthrough etc.

Could it be with with turbo the exhaust valves should be as big as possible while the supercharger should perhaps be the same ideal ratio as used for NA?

The Z18et uses 42mm inlets and 38mm exhausts! while the ca18et uses 42mm inlets and 34mm exhaust strokes are 78mm vs 83.6mm respectively.

So the stroke could also play a factor or bore x stroke ratio?