65 is lower than ideal, but if you're relying on ambient air for that side, there's not a lot you can do. Try covering that end of the viv with plastic wrap or a towel (don't worry, it'll still get plenty of air). You could also aim a heat emitter on that side (I do this with Butter's bin) to bump it up some.
Most importantly, make sure that there are multiple hides between the two extremes. You don't want you snake to have to choose between 2 spots with a 20 degree spread.
Hmm I think Dean must be implying something about how you're talking about king snake husbandry.....maybe :shrugs: