Chasing a lean condition

I need some tips on where to go with my lean condition. I have an 83 2001, 351w with edelbrock performer and electronic conversion. Otherwise the engine is stock. It has about 725 hrs. the issue is the plugs are white as new and I cannot find the source of the extra air. I have checked all around the intake and carb base for a vacuum leak, have changed the carb base plate( throttle shaft leak). The carb has been rebuilt( by me) and the boat runs like a champ, but the plug readings scare me. I get about 40-42 mph and about 4000-4100 rpms and I'm thinking the low mph and rpm may be from the lean condition. I don't run it at WOT except when I am tinkering with the lean condition( I know what happens with lean at WOT). Thought about rejetting the primaries and converting to secondary metering plate with replacable jets. if I do, what jets would be good for the initial trial in the secondaries. Any suggestions are appreciated. I want to get it fixed before summer. Oh yeah It has a 540 Acme prop.

Thanks. Will do this week and let you know what I found. The white plugs scared me because I have always looked for a light tannish coloring near the tip. I did the search on the topic and learned something from you about the soot ring. Will look for it and get back with you. Thanks again