Gotta build ANOTHER engine.

On an SBM, the easy way to time the cam, is to put the #1 piston at exactlyTDCexactly.. Then rotate the cam to put it on split overlap exactly. Then without moving either crank or cam, slip on the pre-timed sprocket and chain set. If it doesn't fit, use a different crank slot until it does. If either cam or crank moves, start over. It is permissible to advance the CAM up to 4 degrees for most cams, thumper-types not included, to get the initialization done.
From this point you are within a couple of degrees of perfect. Now you can install the degree wheel and fine-tune it.
I've been doing it like that on everything that runs a chain, for IDK well over 4 decades. The initialization works on gear drives and Desmodromic too.
After you get this done, you will want to rotate the crank one full turn, so #1 is now at TDC-compression and ready for the distributor.