My C3 spent 24 years in a climate-controlled garage (allegedly.)  Some knowledgeable folks have told me that the grease in the rear wheel bearings is probably solid and not really lubricating the bearings.  I have no symptoms, but I haven't driven it a lot since acquiring it a few months ago.  The worst case scenario is that one of the wheels could seize up and possibly break the spindle, thus losing the wheel.

The question(s) for you is...are they making any noise?
Are you planning on doing any long distance traveling in your '77?
How do the trailing arm bushings look?

I know the trailing arm (front) bushings are tough to see, but if you take the wheel off and peek down in there with a flashlight you should be able to see if they are coming apart. You could also use a small prybar to see if there is any side to side or up and down play in there.

I finally replaced the rear wheel bearings in my '81 last year. They had over 100,000 miles on them. One was making a little noise and had some freeplay in it. Plus I wanted peace of mind on long roadtrips.

I don't think there is anything to worry about. IF it was in a climate controlled environment, the seals/grease shouldn't have been exposed to the hot/cold seasons, and should be fine for use, imo.
If it drives well, and you don't hear any scraping, squeaking, popping noises from the rear, I would think you're good to go. If you just want to be safe, and go ahead and do the trailing arms, by all means, go for it..it would certainly be one less thing to worry about. I would suspect you would have more problems with the rubber parts, such as hoses, belts, and bushings, the the grease in the spindle bearings.