Special B is a great malt to add to these and takes well to extract brewing. A light crystal malt will help with head retention and body and a light addition of chocolate malt could also add some depth to the malt profile. Also, your late boil hop addition is a bit hefty. I like my trappist ales with a lot of yeast character, and I think that much hop, that late in the boil, could compete for attention on the nose.
As far as how much sugar? It's up to you really. The main reason to add sugar is to increase alcohol without increasing body. This is most important for AG brews, whose grain bills, in order to get up to ~7% ABV, can be quite large, and therefore could result in a lot of unfermentables. For an extract brew, you can probably taper it down to about 1 lb, make up for the rest of the fermentables in extract.