LA240Z

Just got a 73 240. It has the N42 head and (arggg) Hitachi's. Right now it runs like a pooch. You need to really wind it out to keep it from lugging. I noticed a slight lope at idle that suggests a vaccum leak. Combined with the carbs and needing a tuneup, I figure that is most of the problem. My question are: 1. I understand that the bronze guides on the N42 can sink, leading to disaster. Is this common/inevitable (do I need to address it like NOW)? 2. What is the most cost effective way to swap out the Hitachis for early SU's ? Thanks.