When you say "Dark Munich", that can mean a lot of things - which are you talking about?
Munich II (8*L)?
Munich 10? Munich 20?
ALso, "between 6 and 9%" is a huge range of final result.
I would be wary of anything darker than Munich II (8L) as 100% of the grist, especially in a high gravity beer, lest you end up with a super heavy, cloyingly sweet mess. Dark Munich can leave a strong maltiness that can be perceived as sweet in high percentages.
By all means, give it a try, but be aware you are likely to end up with a beer you will perceive as heavy and sweet. But maybe that's what you want. I only offer this cautionary advice since you said you were a beginner.
But for the sake of follow-through, please post your final recipe and impressions of how the beer turns out.