WHY low vacuum on freshly rebuilt motor?!

Nothing wrong with the cam choice. How was the motor broke In? Lobes go flat?
What oil did you use on the break in and what procedure?

Get the ignition tuning right first then like cracked keeps saying then tune the carb. If you mess with the timing at all, you gotta go after the carb tuning again No getting around it.

At this point I would make 2 guesses, cam is going flat or the cam timing is off from not being degree'd in properly along with the standard timing and carb tuning.