If it's an 800 I think farah wins hands down. The 800 requires an aerobic ability a sprinter just doesn't have, even the best in the world.
The 600 is a different story, I think it'd be very very close. Bolt has the speed to go through at like 48 but idk what would happen in the last 200. Then again farah does have a lot of speed for a 5 and 10,000 guy. He's closed a 5k (or 10k?) in 50.2! Who knows what would happen.
I think the 600 would be the best distance out if the two for them to race. It'll definitely be fairer than the 400 against Rudisha.
I'd really like to see Bolt accept and lose to be honest. I've always seen him as kind if a douchebag and show off, but that's just me.