Do not use Metapost unless its really really necessary. If you do not
get them straight to start off with you have posts that lean over. Hit
a stone as you drive them in, and they twist. So you have twisted
posts.

Metpost repair spurs slide neatly between the post stump and the
concrete - it would be quite hard to put them in crooked.

the way to get them straight is simple: clamp the post in the spur before
it is put in the ground and keep the post vertical.
This makes a mess of the post top so use the old post then throw it away and
put the new one in when the spur is done.

bland.