Start out with the IBU to OG ratio of the original, then figure out what OG you want to shoot for on the first batch of the partigyle, something like 1.080 or 1.090 for a nice American barley wine that won't be too difficult. Remember that your efficiency won't be nearly as good, assume 10 points less than average for the bigger beer, better to over shoot than under shoot in my opinion. Now that you have that, find out your new IBUs from the IBU/OG ratio in the beginning, if it is in fact an American barley wine you're going for, maybe add a bit more to the hops at the end of the boil since the extra malty sweetness will try to overpower them.
Expect to have worse hop utilization from the hops in the big beer, and better from the hops in the small beer.
Maybe keep a bag of DME on hand in case you don't pull enough sugars from the mash for the small beer. Also some rice hulls in case the extra grain in your mash tun causes a stuck sparge.
As for the oak, New oak has some really strong/harsh flavors. Boil the cubes in water for ten minutes, then change the water and boil again, repeat that 3 times. Also, soaking them in alcohol can help too, since alcohol will pull a lot more flavors out than water. I currently have some cubes sitting in a mason jar of cabernet for a stout.
Don't expect everything to go exactly as planned, it's a busy brew day, rdwhahb
