[Jac Mac]

All of the OE mechanical cams ran 0.025" lash according to a 'Muscle Parts Catalog I still have. I also checked my Comp Cams catalog & with the exception of two fairly radical drag cams they recommend 0.022" lash for their mech grinds, so I think you would be fairly safe to set the lash @ around 0.020/0.025", but if you can find out from the actual builder of the engine in question he might be able to help pin down the actual camshaft used.