In a strictly 'grocery getter' status, a 67 327/275 calls for an AC-number 44 spark plug in an old Chilton's Manual I have. AC web site now recommends a R45S. The resistor (R) portion I can sort of understand in today's world and maybe even the 1-step hotter plug for an older engine. But, what's the advantage of the extended tip (S)? Any?

Putting the spark out farther into the chamber should give a slightly faster burn rate, quicker pressure rise and maybe some improved efficiency.

And with the crappy fuel being offered today, it helps for a more complete burn (lower emissions).

Appreciate it. Been running R45S for a long time with no trouble. Just courious if the 'extended tip' ever caused a problem with touching a piston. Wouldn't think so with factory stuff in a relatively low compression engine.

Extended tips tend to have high average temperature but lower peak temperature.

This is a big deal. The porcelain stays clean 'cause it stays hot, but it doesn't ping because the hottest part of the plug gets cooled by the incoming charge.

Effectively, it becomes a wide-temperature-range plug in the same way that multi-viscosity oil works well over a wider range of temperature than single-viscosity oil.

Your daily-driver is a PERFECT candidate for an extended-tip plug, but you're right to think about interference with the piston or whatever. If it's a recommended plug in the plug manufacturer's catalog, you "should" be OK.