Happy 326 day! Well, it's actually a 336, but, that doesn't exist on the calendar, and Pontiac must have realized there were problems with labeling it a 336, 'cause THEY decided it was going to be known as the 326.

I bet you thought I'd forgot huh?

Pontiac had gained experience with reducing the bores in its standard engine when it downsized the 370-cu.in. V-8 in 1958 and the 389-cu.in. engine in 1959 to 336 cubic inches, for use in certain GMC trucks. It was also possible to put the engine on a weight-loss program by employing thin-wall casting techniques for the “326” and 389 (and even the 194.5 four-cylinder engine) that saved about 30 pounds over the 1962 model engines.

By decreasing bore size to 3.781 inches from the 4.0625 inches, 336.67 cubic inches could be realized while retaining the 389’s outer dimensions and accessories. Any features required for mounting the engine in the Tempest and Le Mans were also added.

Despite the fact that this “new” engine measured 336 cubic inches, Pontiac identified it as a V-326 in marketing materials and on the car’s emblems.

https://www.hemmings.com/blog/article/pontiac-326/

it's my guess that due to GM's rule that NOTHING overshadow the Corvette, they told Pontiac that the number had to be smaller than the Vette's familiar 327. Hense, a 326.... for no real reason that makes sense, other than ego and marketing