To start with a cub likes compression and timing so i would start there. Measure your squish before you just go throwing some smaller domes in it. You are going to have to get the squish where it needs to be before it will run right, personally i would run it between .036-.045 but some may disagree. I have seen pump gas 421 cubs that acted this exact same way and the problem was they were running a squish of about .060 and 19cc domes with about 145lbs, but the problem can come from many different things, did anybody port this cub, are you sure on the jetting, timing, compression ect...
To be honest I think a 421 cub should be running 180+ compression with at least +7* of timing and good gas...otherwise what you could have just built a 350 stock cylinder for a lot cheaper and made 75hp...imo...