Are stainless ARP header studs (1.67") long enough to work on logs???

p-jet

Depends on the thickness of the flanges on the manifolds, on a 3/8" bolt the thread penetration needs to be 1-1/2 diameters of the bolt, so that would be 3/8"+3/16" which would be 9/16" 
dont forget to add the gasket if you run one 
GT