What is this tapping noise? Lifter, flexplate, oil, or something else?

Im no guru, but it could be your tappets. perhaps put in a set of ear plugs - get the noise there and pop yr head under the hood - u should have more accurate hearing as to where the sounds coming from... on the older tappet engines they tended to go out after a while, could just be the extra clearance rattling about - . i think they're very easy to set if u have the experience - im sure u know half a dozen ppl who can - more chevy 350's in the states that coke cans in the world i think :)