What should the valve clearance be on my 1968 Fastback, Small Block? It's been tapping

 

Hydraulic or flat tappet?

 

It's hydraulic

 

zero clearance for hydraulics. Do you have an adjustable valvetrain or non adjustable?

If its an adjustable, this link has some pretty good advice.

http://www.v8s10.org/forum/viewtopic.php?t=81

I've never really messed with a non adjustable valvetrain (always converted to adjustable) so I'm not sure how to fix a non-adjustable valvetrain?

However I have to ask, what sort of oil are you using? Modern oils are hell on older engines unless they are properly formulated for them.