The shop manual says .035 plug gap for a 1981 250.
You could use the original 250 mounts, but they would need some cutting/welding/fab work on the frame side.
The radiator will have to move forward and down to accomodate the 250 fan.
I believe the fan on the 250 will be about 4 inches or so lower than your 216 fan. (measuring from the crank centerline)
