At the last Brew Crew meeting someone brought a keg of Brown Porter that was mostly Brown malt. It was FANTASTIC! I'm planning on getting the recipe.
As far as conversion goes, if you can't find fully converted brown malt, you can use a longer mash time. I'd play it safe and use 6-row in that case.
You may also consider using a pound of rauch to give it an more authentic smokiness.
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