If you want a close conversion to all-grain I'd go:
13 lbs. 2-Row
1.25 lbs. Roasted Barley
4 oz Flaked Barley
4 oz Black Patent Malt
4 oz Dark Crystal
2 oz Chocolate Malt
Then add enough Fuggles or EKG hops to get the IBUs in the 40s, since it's a higher gravity beer. Probably 3 oz, if you're doing a full boil for 6 gallons.
A few things to note, the chocolate has fats in it which will reduce head retention. Plus, it calls for a belgian yeast, which will change the flavor of the beer a lot (fruity/clove belgian esters). But maybe that's what you're looking for?