Trying to start rebuilt engine, spitting through carb

I would be willing to bet that the ignition timing is retarded. I know that it seems that you did your homework on the timing, but it sure seems as the timing is off. Just try advancing the distributor a bit and see what it does. From there the only other thing I can think of is that you have a vacuum leak somewhere. Check all your vacuum hoses. Does not seem to be fuel related, but you never know. Could the idle mixture screws be completely closed?