Lowering Bike height

First thing you should do is set the sag on the shock.

At 6'3", you shouldn't have any issues with reaching the ground. I know a lot of 5'5" 140 lb 15-16 year olds that race 450s comfortably.

Don't adjust the height, it'll mess with the handling. I started riding a full size bike when I was 5'9 just fine, and many other people do it as well. There's absolutely no need to be able to flat foot both feet while sitting.

Set your race sag first. It should be between 100-105 mm. After that check your free sag. If it's between 30-40 give or take a little that your rear shock is the correct spring rate. If your above 40 your spring is too stiff. Below 30 too soft. There are a few things you can do once you have sag set correctly and right shock spring rate.

1) chop the sib frame
2)cut your seat foam
3)lowering link

Based on your height I would say you don't need option 1 or 2. It's likely a geometry issue. Magazines have said that the newer yamaha four strokes ride a little stink bug. Or high in the rear end. A lowering link from ride engineering or any other company will lower the rear end a tad and stiffen up initial curve of the shock. Help keep the bike more stable through chop.

Set your race sag first. It should be between 100-105 mm. After that check your free sag. If it's between 30-40 give or take a little that your rear shock is the correct spring rate. If your above 40 your spring is too stiff. Below 30 too soft. There are a few things you can do once you have sag set correctly and right shock spring rate.

1) chop the sib frame
2)cut your seat foam
3)lowering link

Based on your height I would say you don't need option 1 or 2. It's likely a geometry issue. Magazines have said that the newer yamaha four strokes ride a little stink bug. Or high in the rear end. A lowering link from ride engineering or any other company will lower the rear end a tad and stiffen up initial curve of the shock. Help keep the bike more stable through chop.