Dude, be careful...... 3420 is way too fast even in a 30" barrel. You wouldn't like it to much if your firing pin blew out the back of your action and hit you in the face?
I use Retumbo in my 6.5/300 WSM
as well, it's a Shilen 1:8" 4 groove Ratchet barrel 24.5" long. I get great accuracy with the 140 Sierra MK, and the 140 GameKing, but I don't hot rod my loads, 3000 fps w/the SGK, and 3050 fps w/the SMK 20' from the muzzle with 65.0 and 66.5 grains but they shoot bug holes!
You'll soon discover a very fundamental truth about ballistics: once you have selected the cartridge case and the bullet, there is really no practical gain in wind drift from a load with excessive pressure. You are simply increasing both the danger level and the expected group size. That last point is worth dwelling on for a moment - once a load passes a certain point, reliable accuracy diminishes and that point is usually well below maximum pressure.
Honestly running 250 fps faster isn't worth the danger or the amount of abuse you're placing on your rig. Look at these photos of what can happen when mistakes are made:
The Rifleman's Journal: Cartridges: Long Range Reloading Safety - .308 and Others
Please think of safety first, and bragging rights for someone else.