I did something similar last summer - train to Oban then ferry to Barra, then cycled south pount of Vatersay to Butt of Lewis Lighthouse on the northern point of Lewis.
There's a good campsite just outside Castlebay on Barra (on the west shore).
One thing I hadn't factored in is how heavily crofted the machir parts of the islands are - this means it can actually be quite hard sometimes to find a spot of wild ground on which to camp as most of the flat coastal plains are partitioned up into active farm land - rather than distinct villages it's endless houses dotted here and there.
There's a good bacon roll bus in Leverburgh once you get off the ferry. Do stop in and have a chat with the owner, he cooks a mean breakfast roll and likes to chat about cycling. He wanted me to choose his new bike for him when I was there.
Buy your ferry tickets in Advance in Oban, you'll save a lot of money buying one of the Island Hopper deals over buying lots of single fares and won't be worrying about having to buy them.
The Uists are very flat.
The divide between Lewis and Harris isn't water, but some thumping great hill. Be prepared for some long, steep slogs out of Tarbert. The road at the top has a very alpine feel to it, with snowpoles on each side and a lot of gravelly substrate around. I couldn't see much because of low cloud when I was up there, but made it even more atmospheric.
Take the east coast road on Harris.
Make sure you go to Barra Airport and watch the planes land, then go to the little chapel and graveyard on the road beyond.
The people of Lewis drive like kamikazes, be prepared for this.
Be prepared for endless headwinds and beautiful views.
Shops can be far between but even the little ones are very well stocked. Call in at them all on your way past and make sure you carry extra food (especially if camping) as you may not pass somewhere for quite a while.
Stornoway, Schmornoway, didn't really have much to commend itself to me.
Make full use of the various power sockets on the Calmac ferries to charge your phone etc. Be first on the ferry and hunt out the table next to the socket and set up camp there.