If you are already down that far it probably easier/more accurate to just to pull the cam. But since you asked, here is what I would do if I had oo.
Put a degree wheel on it and a dial indicator on the lifter. It would have been easier to measure from the rocker while the jugs were still on. The fact that you can't use the correct checking clearance will throw you readings way off. On second thought, it might be easier to bolt up a jug and rocker to one side.
Here are the specs on the 336 cam.
The specs for the stock is here.
The two cams are radically different enough that you should be ablet to tell that you have something other than stock. Do not expect to reproduce the published numbers. Google "degree a camshaft" to get a general idea of the process.