Spark plugs

Just repeating what I’ve heard, maybe try 17 lbs air in tire. The has

I have in my 85 HP Continental with Auto-lite M40E plugs set at .015 but I'm told there is a better plug that will improve performance, is there? If yes, who sells them and how much? What gap is needed? Also I have a shimmy on landings, not taking off, I was told the end bushings need to be changed and that's a big job, others said "maybe a nose wheel out of balance" so how do I know for sure before I pay big bucks to a shop to replace those end bushings.