As it aged, the roast character became subdued so the flavor profile got more like a barleywine (if you have had Flying Dog Horn Dog, it was very similar after about six months). To counteract that I'm going to double the chocolate to 1# and sub some special roast for some of the C60 when I brew it again this weekend.
Malto dextrin you won't need, there is plenty of body already. The patent/black malt would work, just don't use too much or it'll turn into an imperial stout.
That'd work great.
Mash a few degrees higher if you want it to be more dextrinous. It's already pretty full bodied though.
Cali Ale V and London Ale are my two go-to strains, Cal V I would mash at 152-154*F, as I mentioned I'll be using London Ale at 150*F to achieve a similar attenuation. Either yeast will produce a fine beer as long as fermentation temps are controlled.