All you really need is the tunnel "flange" from the 300 seatbox, so if you old one is in good nick, a rusty one will provide the pieces you need. Drill out the spot welds, attache the flange to your seatbox with adhesive and rivets. Trim out your seatbox to match the tunnel, job done.
Also, if you don't need a middle seat, the hand brake form a RRC can be used on top of the seat box, gives much needed ankle room.