Hello Barkingnut,
Some help from a Froggy ?
Correct U2/U3 files :
http://www.gametronik.com/site/fiche/pinmame/Time%2520Line/
You'd like an easy way to change your backbox ?
https://www.flipperfrance.com/threads/7-digits-kit-adaptation.12361/
You will preserve the original wires and easily come back to original 6 displays version.
You also will able to mix 7 and 6 digits displays on the same backbox.
A modern CPU ?
http://www.flippp.fr/pi80.php?lg=en
Send pictures when you will succeed.
Regards
TMR