Car stories....and I mean stories

Is this a test?

Changing the engine would have no effect on the speedo.

Changing the rear end gears would, but you said the donor car (Newport) had 2.76, but the GTX had 3.55.

Why would you change the rear end if the engine was blown?
Wellll, the speedo drive gear on the trans output could havve a different tooth count and affect the speedometer. Of course checking as the colour was generally different for different tothh counts and then pulling the tail housings to switch gears is quickly done.