Some icky remarks in here. I was that heavy when I got my PPL.
I never needed a seatbelt extender.
I never stepped through any wing walks.
There were quite a few planes that would need aft ballast. 5 gallon water bags from walmart can work for this, at ~30# apiece. Sometimes you need partial fuel to make it work.
I've never seen a published max weight for wing walk or seats.
You should teach your student to step over flaps where practicable. Sometimes this means dropping a knee onto the wing walk to avoid the flap, then pulling self up with a hand on the spar line.
You should teach them to NEVER bear weight on any cabin door. They should grab the roof structure and lower a foot into the footwell for a low wing; for a cessna, it's just a matter of leading with the inboard foot and kicking off with the other foot using the cabin step.
For any new plane, set the seat to the rearmost stop, check for vertical adjustment (crank it down), and recline adjustment (a notch or two back does wonders), then tailor to fit from there.
The comfort for big pilots seems to favor, in order: beech, cessna, piper. It seems to follow the size of the men behind those names.