450r A Arms WONT work on a 300ex.... without modification. The frame mount side is too wide. In order to work, you have to cut off the a arm bearing housing, and move them closer together. It has been done, and can be, but you need a good clean cut, long threaded rod to hold both a arm end housings at the right distance apart. Tricky to get it right, but it can be done.
Ive got a complete 400ex front on mine now, and it is fine. I was worried about the negative camber, but rode it, got it on a level floor with both fronts at 30psi. Its not that bad. Sitting at about negative 3.5-4 degrees. "Heard" the 300ex spindles work better, but... 450r front shocks on it(diamond J) dialed in on first try. Hugs and no more tippy feeling.
Cant comment on the swing, got a steal of some ITP offset beadlocks for the rear with the 450r front shocks. Widened the rear, no need to extend it for the wife =). Just gonna slap a 01' raptor 660 rear shock on it and call it good.
BTW, I do have a set of 450r a arms I need to sell(cheap). Balljoint seals are not perfect on all, but if any one wants to try and mod for a 400ex or 300ex, might as well go all the way, and mod the balljoint sides to accept aftermarket balljoints. This has been done also.
If you are, or know a good welder/or just sharp with figuring stuff out, anything can be done. Fabbing stuff that "wont" work is the a fun part of the hobby. My Yamaha Blaster has 450R shocks all the way around, GSXR rear brakes(not modded by me), and YFZ 450 heel guards =).