No need for different chain, sprockets are the same. You can move sprocket & carrier over from original wheel. Then use chain adjusters on end of swingarm to adjust before tightening axle.
Shock on 300 is same length as 250. Make sure you don't have knuckle upside down. Take photo from side below.
Did you grease all pivot bushings?
Not sure were you needed washers?
Get rid of that Crescent adjustable wrench!!!
Only use sockets with torque-wrench to set proper bolt-tension on any suspension-related part. These are critical and can be difference between life & death!!!