On the 763xm the F/K seats are not offset like they are in the forward cabin of the 77L/77W. Your only real choice is do you want to sit in front of each other (eg 4A/5A) or across from each other (4F/4K). If you choose to sit across from each other, make sure it's F/K not A/F since there is a wall only 3 - 3 1/2 ft high on the left (facing forward) side of the F seats.
One other thing... just to make sure you knew that there are NO provisions for bassinets in XM J.