If it's the original engine it's an easy question. It's the "Z" code 300HP version (the Z is found in the Vin on the door tag).

There were no optional or special order engines available in the 61 Birds.

The 62/3s had the optional "M" code engine available. This was the higher compression Tri-power motor.

The Tri-power is a nice upgrade to the engine, but if you go looking for one make sure you get the T-Bird version as it's shorter to fit under the Thunder Chicken hood and the 406 versions won't let the hood close.