sounds like you may have a mis match of parts-------caution area----a external balanced crank-probably is for a one piece seal block? if so , the bolt pattern to end of crank is different than the internal 350s are-------------if it is just external balance and has the early bolt pattern, use a 400 sbc or 454flex plate--its external and right bolt pattern---------if you use one of those spacer bobs-and weld on it, you will change the weight by about a gram per inch of welding rod.


so please clarify what you have with some part numbers and pics