I haven't built any of the Atomic Zombie designs, but I am a fan of his work and I read his blog and forums often. He often gets kudos for clear and concise plans.
Looking at pictures I can see that the seats are a couple of inches above the axle line. With 20" wheels, that would put the axle line nominally at 10" above the ground and the seats around 12" off the ground.
If you need a seat that is similar to an office chair you will need to raise the seat about 6" (my office chair measures 18"). Using 26" wheels will raise the axle and frame 3", and you can then build the seats another 3" taller. Taller seats will change your angle to the pedals, so you would have to slide them forward to accommodate that and that would give you a little more cargo space behind the seats.
That would give you a floor height of 13". If that is too high to step in, you would have to redesign the frame to give a lower floor. I would post on his forums to see what the people over there suggest.
Good luck and keep us posted on your progress!