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The A12 engine is very smooth and quiet. This high idling is usually caused by a vaccuum leak (unless your carburetor is shot).

Examine carefully all the vacuum hoses on the engine, especially the ends of each hose. Plug temporarily the hose going into the PCV valve (in the manifold, just under the carb) to see if the engine slows way down. Also plug temporarily (as a test) the hoses: vacuum advance on the distributor, the small hose going to the air cleaner, and the hoses going to the circular diaphragms on the carburetor. Just put a golf-tee or something to tightly plug the end of the hose.

If nothing shows up, spray the carb-manifold junction and the intake manifold-head junction with WD40 -- if is smooths out while spraying a particular spot, tighten the bolts or replace the gasket. The idea here is that the oil temporarily plugs the leak and you can hear the engine run differently.

The TDC timing mark is almost certainly the biggest mark of the five (the last to line up in direction of rotation). If there is a bigger mark in the middle, that's the stock timing mark. Newer engines had the timing marks on the front cover (clearly marked with numbers), with a single mark on the crankshaft pulley.

If your camshaft is off a tooth, that might cause very rough running ... have you changed the cam or had the engine apart?