That makes sense. 405 is what I was finding in a look-up for a 273. But it was lumped with the 318.
Funny that Bewy recommended NGK BP6ES.... I actually have a bunch of those.... use 'em in my 1966 MG.
Thanks, everyone, for the background and recommendations. The car has been parked for 9 years, so it has been a while since I have gone through a tune-up.