The T-Birds were always popular to cannibalize, as 1961-1968 were all 390 or 428, early '68 was 390 or optional 429, and mid-'68+ were 429 as standard. Look on the edge of the driver's door for the warranty plate. The 5th digit of the VIN is the engine code. M-code was the 3x2 carb 340hp 390, Z was the 4v 300 to 315 hp 390, Q was the 345hp 428 (in '66-'67 only), and N was the 360hp 429.
While you're peeking, look at the warranty plate for the rear axle code. If it is a letter code (not number) then it is a locking rear axle and more valuable. T-Birds had them more often than other models. Have fun.
David