The car is worth saving, but it will be a lot of hours and several fists full of money to do it.
I would say it's probably a $15-$20K car maximum.
Cooling system: $1.5K
Exhaust: $1K
Engine rebuild: $3—10K
Trans: $3—5K (plus 6 months wait)
Windshield: <$1K
Steering wheel: nice non-stock $300 (stock rarely available)
Carpet: $600—$1K
Repaint: $4—10K
All the stuff you don't know about yet: $5K Trust me, there will be LOTS of stuff that you can't see yet on a car that has been sitting that long in a barn.
Hours: estimate 1000 if you do most of it yourself and want a really nice car at the end and are really handy mechanically.
It'll be a great way to spend your time and just think of the money you'll save not going to movies, restaurants, vacations etc...
I hope you go for it, but DO NOT pay $30K for it. I would offer $12K and promise that it will re-live it's former glory as your mom's cousin would want. I doubt if on the open market it would get past the mid-to-high teens as-is.
Good luck!