I am close to your size (5-8, 155 pounds) and paddled a Nomad 8.1, which is comparable in size to a Mamba 8.1, for a few years. I thought the sizing was about right. The 8.1 is not a big boat by any stretch of the imagination, especially compared to newer creekboats where the trend is bigger is better. In addition, I've seen lots of guys - and gals - who are about your size or smaller paddling Nomad 8.5s or medium Newmads that are slightly bigger than the old 8.5.
In short, my guess is that a Mamba 7.6 is WAY too small for you. Hope this is helpful to you.