recommend fin for MR single fin 6'4"

I’m still using the Rainbow tkflex 7.5 raked fin that was the only single fin the shop had when I bought my board. I’m in beachbreak which is fairly slow most of the time. I have the fin placed fairly far back to try to get more drive. Board turns ok, a little better when I move the fin up but then I lose speed/drive. As the thread states it’s an MR 6’4". Any recommendations to add speed?  

http://trueames.com/shop/parrish-single-fin

Thanks for the link. Is 7.5 the size I want? I’ve been thinking I may have too much fin?  

depends on what 8 inch fin you have .

 

That parish fin is what i would use

 

this is the board —> http://www.markrichardssurfboards.com/boards_retro_1979single.php

Yes that’s the board. Sorry I edited my post. The fin is a 7.5 rainbow flex.