I haven't looked at the cabin in a long time but I seem to remember the joists running over the tops of the beams? If not then the longer spans can create problems in any notching of the underside of the ends of the joists. There are some "relatively" simple calcs that can be made to determine the allowable notch size or you can "scoop" the material out and gradually diminsh the timber size to your beam pocket's depth... within reason.

By the way, if you're using pine and it's a first floor load as Jim defined above, the 4x8's would work, but they're bouncy with a 1/2" of deflection fully loaded.