Yeah I agree that is why I asked him whether he actually ran and threw to the fullback. If you are using him strictly as a blocker then it would not be worth it surely.
But if you are drafting him with the intent of using him as basically an RB2 that lines up at fullback, it doesn't really matter how good his blocking or what his OVR says. If you set the type to "runner-receiver" it doesn't weigh blocking as heavily on the OVR either. But even if it's low I'm not sure it would matter.
You could also just leave him as a RB and not even convert him also (but just play him at fullback)