The bolts can become fast due to burrs on the washers. I used this method to remove mine:
1. With nut slackened but left on the end of the bolt for thread protection, use a lump hammer to send the threaded portion into the casting and expose the bolt head at the other end.
2. Insert two adjustable spanners into the exposed shank (see photo), with spanners set just wider than the shank diameter.
3. insert progressively larger wedges, using the spanners as levers, to increase the exposed shank length. I used a screwdriver then successively bigger bits of wood until it finally came out - many iterations.
4. Don’t forget to deburr the washers prior to refit and use the same shim pattern.

Wedge removed to show spanner arrangement.

 

The washer and shim holes are very close to the shaft diameter.
Reaming slightly will help with installation.