I think it was the chunk in the front of the trans that was different. I'm not sure about how many 340's were externally balanced but I do think it was only the one's with cast cranks. As far as I know all the 360's are balanced externally, they replaced the last 340's and yes all 273LA's And 318LA's are internally balanced. I believe the early 318A's all had steel cranks with internal balancing as well:banghead: