Very nice!
Don't trust the stamping in the tunnel....both of my '79 Invaders show SS-440-A
The foil sticker at the back of the tunnel will show SS-440-A2...if it's a '79 and it is still legible.
Also the build date will confirm...
2 differences between the 78 and 79 hoods:
The 78 Intruders and Invaders used fiberglass....the inner surface will show a rough appearance typical of fiberglass construction.
The 79 and later hoods have a smooth, gray finish.
I don't think the 78 hoods had the red pinstripe around the black headlight area....just the gold.
The rear suspension changed from 78 to 79.
The front arm spring adjustment on the 78's was a threaded eyelet bolt that you adjust the nut to change spring rate.
The 79's and later have a cam mounted to the skid frame and you change the spring rate by moving it to different notches inside the tunnel.
You are right.....that engine is toast
....hopefully the extra one is usable.
Good Luck bringing it back to life!