You'll need a mountain type rear derailleur to handle a cog bigger than a 30. I agree with the others that starting the cassette with a 12 or 13 would make more sense but don't know about availability in 8-speed. Sheldon Brown could probably help you with individual cogs so that you can create your own cassette. Converting to 9-speed could also be a good option but would require new 9-speed shifters, chain, as well as 9 cogs in the back. And there are 12-34 mtn. cassettes.
Al