i'd give the 5's a go first,the difference between 5's & 6's doesn't appear to be much in bp_es',so try the hotter plugs first.if there is a problem,then try 6's or maybe even 7's,i used to use bp7ev's in my engines,with no fouling problems,except when a leanout chewed the side out of number 3 piston one time.i cheated my way round that by swapping plugs cylinder to cylinder,so the fouled plugs cleaned themselves up.it got me from tamworth to sydney,which is what i needed.