Aloha, man this sounds exactly like what I've been going through with my wifes '54 belair/235. I finally replaced the carb with a known good from another engine, even replacing the phenolic block that the carb sit's on, and that's what did it, the block was good but I had a leaking throttle shaft and it would not start at all, backfiring up the carb, etc. If you have another carb laying around try it. The backfire up thru the carb could mean tight valves but more than likely a lean mix which could validate an intake leak. I know you think "but it ran before" but man at this point try it. As for adjusting valves I get them close then do it while it's running, you can actually feel it come alive that way.
