i weigh about what you weigh ..
have used an 8.5 on a JP SLW92 with a 53 cm Curtis fin ...
have used an 8.5 on an AHD FF 165/80 cm wide with a Select Freeride 48 cm fin
as stated earlier ... as the width goes down, it seems we use just a slightly shorter fin
now if one wants a fin with a bite - guess slalom is the way to go
these come in tuttle and deep tuttle - so why convert to PB
unless you are worried about the sandbars ??
try a Wolfgang Lessacher weed fin - now sold by Unifiber ??
personally NOT a fan of weed fins
BUT am a fan of PB - when I hit a rock - NO damage to fin box or board !!
lost and cracked a few fins tho