To get a seriously distinct chocolate flavor in a recipe of this magnitude, you might need to use some actual chocolate
. I haven't toyed with an imperial chocolate stout myself, so perhaps someone else can speak to that.
I'd say you should be able to just toss 8-12oz of oats in with the remaining grain bill for mashing. However, as I recall you need to mash a base malt in with them for proper utilization; perhaps throw 1lb of 2-row in alongside to compensate?