I am building up a dynamo hub and need a rim that will be able to take both a CX tyre [ 35 mm ish] & mtb
I have the chance to get a Stans arch rim,internal 26mm external 29.3mm
The rim will be used for bikepacking so I want something pretty strong.

Anyone any experience of mounting a CX tyre on this rim?
What size /type of CX tyre did you use? Tubeless ok no burps? Did it ruin the tyre profile stretching it?

I’ve got cx tyres on the older crests. Im not sure id want to go wider as the wall of the rim is quite exposed even with 38c tyres.

Id look at road rims like dt r460 as they’re generally heavier (tougher), and not a million miles off the weight and profile of mtb xc rims of a few years ago like the dt x470.

Not a simple answer.
Different brand’s tubeless CX tyre carcass widths vary significantly.
We’ve seen 33mm Challenge TLR tubeless tyres fit and ride nicely on 25mm internal MTB rims so I’s say the 0.5mm per side extra width of your proposed rims would not materially change things – they would work nicely.
But Challenge’s 33mm TLR’s carcass width is nearly 1cm wider than a Schwalbe 33mm carcass…
I’d not put a 33mm Schwalbe on a rim that wide.
This is the result of a project looking at Tyre/Rim combinations for UCI CX Races where +33mm tyres are not allowed to race; https://cyclocrossracer.co.uk/blogs/news/uci-race-tyre-project
We didn’t measure 35mm tyres but would expect to see similar variations if we did.
So in answer to your question – width wise, I’d be happy riding Challenge TLR tyres on Arch rims.

I built up a set of MK3 Crests on my wife’s Full Monty with 38c Schwalbe Marathons with no issues 23mm internal old Arch EX is 21mm I think

Not quite CX tyres but I have 27mm internal rims with specialized sawtooth 42mm fitted tubeless, they are a good shape and behave well at low pressure. I would be happy to go wider on the rim/narrower tyre still as long as there is a little tyre proud of the rim to protect it…

I have a similar wheel to the one you are building. SON Dynamo hub with DT Swiss XM421 rim: 25mm internal, 30mm external.
It has been faultless for riding mixed terrain with. Tubeless with tyre sizes ranging from Vittoria terreno Dry 33(35mm) and Jack Brown 33.3 to Rock n Road 42mm.
You will get a very slight change in tyre section, but it’s highly unlikely you will notice it.