I have a large shed with 6" x 3" timber uprights resting on a stub block
wall. The bottom 2-3 feet of the posts is rotten so I want to splice-in
some new timber. I can't think how to cut a decent lap joint in the
bottom end of the remaining upright so I'm tempted to butt join the
timber and add steel plates (fish plates?) on each side - at a guess
these would be around 150 wide 450 long and a few mm thick.

Suggestions welcomed for how to cut a lap joint on the bottom of a bit
of timber that's waggling in the breeze, or for a source of suitable
steel plates.