Pontiac's Tempest went through a major transition in 1964. No longer a shirttail relative of the Chevrolet Corvair, it was fattened up and marketed as a family car. The "sporty" LeMans model featured standard bucket seats and unique trim moldings, but had the same engine combinations as all other Tempests.
To enhance their image, the product development folks at Pontiac Motor Division wanted to install the 389-c.i. V8 from their full-size cars into the LeMans. However, GM had a corporate edict that imposed a maximum engine size for a car of no more than one cubic inch for every ten pounds of weight. The heaviest Tempest crossed the scales at 3,260 pounds, so the 326-c.i. V8 was the largest allowed.