You might have to adjust the mounts alittle on the front, I think the difference in the 450 shocks are like an 1/8"(wider) between the two shocks and around 2.5" taller. The 450 shocks will lift the bike more than the 400 shocks with a lift. I ran stock springs on mine before I installed a complete 450 front end on my old 400. I changed a-arms, shocks knuckles, and axles. Like I said the 400 rear shocks on front will be all you need and a set of 450 rear shocks for the rear of your 400. If you install the lift kit with the 450 front shocks or 400 rear shocks on the front end, it will put the front end in a bind. You can install lift springs and it will be fine.