64's never came with a 429. First year for the 429 was 1968. The 67's offered the 428 for the largest motor. I would conclude that someone dropped the motor in the '64. 429's were used from '68 to I believe 72 or 73. Then came the big 460's. I might add though--- the 429 was a good motor. Hardy and strong, ran forever. Plenty of power. Especially when pullution started to play a bigger role in engine performance starting in about 69-70.
Absolutley and without any exeption the only engine available in the 64 t-bird was a 390 300 hp. If there were any other engine in there, it was transplanted and not factory installed. 1966 was the first year that an engine option was again offered for the t-bird.
you are right, only the 390 was available in the '64 model. In 1966, the 390 was standard and a 428 became optional. This continued in 1967. In 1968, I believe the 390 was standard early in the year but the 429 was phased in as standard around January 1968. No '64 T-Bird has a 429 as original equipment.