Unless you are using a long bolt with the head sticking out on one side and the nut on the other, then it makes more sense to me to use the bolt off an xocet or omega https://www.gunspares.co.uk/products/24558/Xocet/ B1286 part number and get a threaded top hat insert made/ fitted for the right hand jaw and the left one recessed. At least then you are only messing with the jaws and not having to mess with the breech block? If you dont mind bits sticking out, then a 1/4 inch diameter shanked bolt and nut would do--------but you would have to take a scoop out of the stock for where they stick out.