In my opinion you'd be better off making a set of rigging with the top terminals swaged on and then the wire left long and open ended. Attach all the pieces and step the mast using halyards to hold it up. Add a few temporary extra halyards just to make it a little more secure by hauling up double blocks with the ones you have. Then with the mast in place, use mechanical fittings at the deck.
It's hard to get exact measurements for rigging even from sister ships. With the backstay, I would just leave the splits long and open.
I can't believe that someone would steal another mans rigging. May it bring them bad Karma.
Good luck.