Especially if they're used hubs, it's smart to go through them with your own eyes to make sure they are in working order and they are ready to go when you install them. Sure, it can be a little nerve racking, but you'll get them done.

As for the allen bolts, try using a little PB blaster or penetrating oil to losen them up. Use a ratchet and socket with the correct allen size head. You'll get better leverage. Is the tire on the ground or is it up in the air where it can spin when you try to tighten? You can apply a litle bit of anti-seize to the bolts before you re-install them.

If you really can budge the bolts and the hubs are off the truck, place the hub in a vise, then you can apply some force with two hands to the ratchet. Or you can use a hammer to gently persuade (tap) the allen key.