You might discuss this with your father-in-law first, just to keep peace in the family. Z's kept that long are loved and he might cherish it more than you - or he - realizes.
Assuming he's cool with it,
http://www.BalckDragonAuto.com sells a Racing Rollbar that is SCCA approved. It has the engineering done for it's designed use and includes attaching hardware.
But be forewarned it is a rollbar and not a roll cage.
As for collectability, these cars are not rare and have little potential for increasing in value after restoration - but heck, people restore '58 Chevies.
220K km is not alot for a Z engine and transmission. After some re-conditioning, you could use this car as a daily driver for several years and enjoy it while it's still here.