I had a 68 Ford Thunderbird and I remember it having a 455 Thunderjet. Am I mistaken?

The engines offered by Ford for the '68 were: 390 cu in (6.4 L) FE V8 428 cu in (7.0 L) FE V8 429 cu in (7.0 L) 385 V8.... but somebody may have stuck one in but it would have been a big hassle because it is a truck engine

The 390 special V8 was standard until 1/1/1968. The Thunderjet 429 V8 was optional until 1/1/1968 when it became standard equipment. All of the competitors offered higher displacement engines as standard equipment for 1968, which included the Cadillac Eldorado, Buick Riviera, Oldsmobile Toronado, Imperial Crown Coupe, and even the T-bird's sister model, the new 1969 Continental Mark III, introduced early during the 1968 production year.

Only 455's I've ever heard of are GM products, Olds, Pontiac & Buick.

These are the only engines available in 1968 390 cu in (6.4 L) FE V8 428 cu in (7.0 L) FE V8 429 cu in (7.0 L) 385 V8 No record of a 455 unless you planted a big block GM in it.

Ford, in circa 1956 manufactured a 455 HORSEPOWER 460 cid but never a 455 cid, except for a heavy machinery application never used in passenger cars or trucks

you are miss-taken, its a 429 Thunderjet ! The 455's are Oldmobile !