Ask friends or big guy surfer who can give you spare fins , you know sometimes people loose one fin of the pair and get spare one that way which can be give for free

so put the larger one on your back side and the smaller on your front. The larger one can be a MRx as riverider said and the other H1 or any other large fin.
For the rear one of your back side remove the GL to a M5, so it will be stiffer on your backand because we use to get more stance on backfoot surfing during backside surfing.