I was planning on making 35L of 1.070 ish OG dubbel on Friday. I don't have any yeast to hand and was wondering whether the smack packs of http://www.brewuk.co.uk/wyeast-activator-3787-trappist.html would be appropriate at, if so, what pitch rate would be best? I use brewtoad for recipes and it's suggesting >300 bn cells are needed, which would make this pretty pricey yeast order. Better to leave for another time and make a starter, or would under-pitching this be OK?
When can you get the yeast? I would definitely not go for it if you're not going to be able to let it get started at all. If you can get the yeast a day or two in advance and get it partially started then you're more likely to avoid off flavors. Even if you have to cut it a little short. You might try buying a couple packs and giving those a short start.
However, let's be clear the best beer will come from waiting for a proper starter. Looking at the BJCP style guide for the dubbel doesn't look good. Under pitching you're likely to end up with more fruity flavors and less of many other characteristics, like body and malt, according to this article at Science Brewer.