410 is fine if you don't have a 415 motor gear and what most kits come with, just use an HD Z-chain, the BMX style, they are pretty tough.
I have started using this Permatx Ultra Copper High Temp RTV silicone gasket maker, it is gas resistant and remains slightly pliable.
It likes like this when applied on a CNS.
I have a topic here about it, 'Inside the CNS Carb'.
Personally I've never had an air leak issue on an NT carb with no sealing at all, but I can sure see an O ring in there being worth the extra buck.
You must be referring to NT carbs not having an O ring as CNS carbs come with one, it is just too darn short to do the job so you need to seal them up at the groove on top.The leaking carb/manifold is a common and well-known issue and it's easily remedied by a simple o-ring placed inside the manifold.
I have started using this Permatx Ultra Copper High Temp RTV silicone gasket maker, it is gas resistant and remains slightly pliable.
It likes like this when applied on a CNS.
I have a topic here about it, 'Inside the CNS Carb'.
Personally I've never had an air leak issue on an NT carb with no sealing at all, but I can sure see an O ring in there being worth the extra buck.