I read the title wrong at first. Thought you meant Stone henge, big ruddy stones them!
I assume the stone to be laid in dry stone config? So when you remove them and soil for your cutting through you `ll have big ugly ragged ends showing.
Could be your cue to learn how to dry stone wall, you will have to tie it in or else as you rightly say bodge leads to ...
The concrete you propose will now be the water break? So have you ever seen water higher than your new step height?
Any timber in the construction best be tanalised, else you`ll be changing it every couple of years. Wouldn`t bother with met post just concrete tanalised into place or use god father posts with tan. bolted to.
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