There is another factors to take into account.
-Are your rear springs currently holding your frame at stock height? (No sag?)
If the amount of sag is equal to the increase in tire diameter, your castor will be fine.
If you have no sag, you could add lowering shims similar to what GM used on the passenger side axle on 1953 pickups with the optional driver side bed-mounted spare tire. (That shim countered the additional sag on the driver side rear spring caused by the weight of the spare tire and bracket.)
Carl
