Most if not all weed fin sizes quoted are the veticle size. A 30cm weedy looks quite big.
As far as what size would you need for you JP V 60. If its 60cm wide its close to my Iso86 and that goes great with a my 30 and 34cm weedys. Both these fins started life as 52cm techtonic mirage pointers until they met Elmo's hack saw.
As far as the the bases go. You cut them off keeping the fin blade along as possilbe on a 45deg angle. Then the blade is held secure in your home made jig and the base of the fin is supsended in the mould. just pour in your epoxy resin/filler mix and walk away (remembering to spray the release agent inthe mould first)
Next day pull your new weedy out of the mould, custom fit it to you board and tap the two screw holes and go sailing.
The hardest bit is making the mould and jig. Once that is done is really easy and yo start looking at big pointers and consider attacking them with a drop of hack saw