I think that the comparison I read may have been between '55 models. The Corvette had a 265 V-8 (it was Chevy's first year to have a V-8), while the T-Bird had a 292 "Y-block" V-8.
Only 700-some '55 Corvettes were made, as there were many unsold '54s, with straight-6s, left over. Meanwhile, Ford sold over 16,000 Thunderbirds. So a 23:1 sales advantage.
The Thunderbird was intended to compete with the Corvette in the marketplace, but it never placed the emphasis on all-round performance and handling that the Corvette began demonstrating starting in 1956.
To be fair to McNamara, the 4-seater '58 T-Bird outsold the 2-seater '57 model by a large margin. So, counting only dollars-and-cents, making the switch was a smart move. What the decision said about Ford Motor Company is a more subjective thing to assess.
But it's that same bean-counter mentality that McNamara took with him when he became JFK's SecDef, and with which he managed the war in Vietnam.
Sometimes cost-efficiency is not the most important thing. It doesn't matter how much money you saved by micro-management if you lose the war.