Well, you'll want the best R1 barreled action in the lefty stock so start with a visual inspection, then shoot them both. If you have a chronograph check the velocities as well.
1. If you're lucky the best looking, best shooting one will already be in the lefty stock. DONE!
2. If your just a little less lucky and the best looking, best shooting one is in the right hand stock, switch them. DONE!
3. If the stars just won't align and the best shooting is the worst looking, switch the guts and put the now best looking, hopefully best shooting one in the lefty stock. DONE!
4. Same goes for the hardware. Keep the best.
Can you keep two? If you're an R1 lover like me and don't already have a 10m match gun, that Walther will be a nice change of pace. MDriskill got me into 10m guns years ago and my R1's hardly get shot anymore.