ZFR5N plugs too long?

The BP5FS is the listed plug for the slant six. The consensus of using the ZFR5N plug is to get the gap further into the cylinder, for a better burn. My personal opinion is that I have seen no change in performance/mileage.
PS: I haven't tried them on a 170 engine, but I would be careful, as the piston comes up to the top of the block, and MIGHT hit the plug. 198/225 engines do not have this potential problem.