Does anyone make a hardware set (nuts/studs) for the intake/exhaust?

I think some of the Z vendors sell hardware ‘kits’. 
 

Not sure if it is still possible, but I sourced everything at my local Nissan parts guy at the dealer. 
 

I’ve also picked up some nice high grade stainless hardware at a local fastener supply house. This option is more work, because you need all the dimensions and such, and the better quality hardware will cost more.

Be sure to get the correct washers for the studs that share an intake ear with and exhaust ear on the manifolds. If using a header, get a good one with the thick flange.

Zcardepot has kits IIRC

Is one of these kits what you're looking for?

Motorsport! Head/Manifold Hardware Kits - The Z Store, Nissan-Datsun 240Z-260Z-280Z-280ZX-300ZX(Z31/Z32)-350Z-370Z Parts