got the two cross pins out, right?

if so, place a block of steel (both pieces should be the same height) on either side of the fsb.

these pieces of steel straddle the press table. they would probably resemble those two black pieces of steel in your picture.

you can use rubber bands or tape to keep them securely under the rear edge of the fsb if you feel like it.

now...fixtured and ready to press, the rifle is 'hanging' by its' fsb...butt down/muzzle pointed up.

place a scrap piece of brass or aluminum on the muzzle crown and then bring the press ram into contact with that block.

wrap the fsb in a rag or use old plywood as a scattershield...and put on your safety glasses.

carefully, build pressure and push the entire rifle towards the floor. stop pressing when the muzzle crown is flush with the fsb.

allow the press ram to travel up and unclamp your setup.

it should be a simple matter to wiggle of the fsb with a twisting motion or tap it off with a sofr wood bock/hammer. in the worst case, a hardwood dowel place against the muzzle crown will allow the press to finish any needed travel required.