WHY low vacuum on freshly rebuilt motor?!

Did you readjust the carb when you increased initial? If not, you're not solving or getting to the root issue. What is the current idle speed?

Is the carb dripping fuel in from the boosters at all?

That should pull ~11" and idle at 800-850.

If you reach in and twist the distributor CCW and it picks up RPM, the initial wasn't right before. Set it at 20* and see if it kicks back on the starter, if not, then tune the carb. If it does, pull out 2* to 18* and see if it kicksback. If not, then tune it. Find the max point the starter can handle.

That cam is NOT big at all. I have a 340 with a 284/528 mechanical, tight lashed at .020/.022 and it idles right at 750-800 with 11-12" of vacuum. It has 24* initial on it and fire at a flick of the key.

Since you know it seems to run OK at 36 total, find the happy initial place and do some basic math to get to your 36 number.