The rear end of my C3 is sitting 1-1/2 inches lower than the front. It would suggest that my rear leaf spring has sagged and needs replacing.  The one that is on the car is solid and not actually leafs.  Ekers and Zip corvette are selling these fiberglass rear springs in different weight ranges around $500. Any recommendations on what I should buy? 

Replaced my metal spring with a fiberglass rear spring in 2013.  My old one had the same issue as you're seeing with the sagging.  Since we were upgrading the entire suspension system to HD and poly, it was planned to replace it anyway.  We were building it specifically for mountain driving so installing everything performance-rated was important. 

They do come in different load ratings as you say, most often anywhere from 315 lbs to 355 lbs.  The one from Ecklers is 400 lbs.  Depending on how you drive it (just cruising, twisty mountain roads, straight interstate, etc. etc.), will determine what you buy.  A higher load rating will give you a stiffer feel, possibly even too stiff if you like a smoother ride.  The ride could be harsher on rougher roads.  Normal rule-of-thumb is for all around performance, the 340 lb is the choice.  355 to 400 are considered heavy duty and 315 is more or less standard.  Haven't been to the New England area of CT. and points north since living there 50+ years ago but I suspect that there are some nice "twisty's" up there to enjoy.  The 315 lb will allow more lean, probably too much, while the 355 - 400 could make it skittish on tight turns. 

Whichever you go with, it WILL be a marked improvement from the stock metal one.  To that point, Mid-America Motorsports has the 315 - 355 kits complete for $425 and Corvette Central has the same kits for $391. 

Under normal conditions, the rear leaf spring should be basically flat with maybe a slightly up arch.  Don't remember when they switched to the fiberglass rear spring though.

Thanks, great info.

Mine is basically flat under the cars weight. That is why I thought it was bad. Not sure what else would cause the back to be lower. Shocks are only a year old.

Maybe you can raise the rear with the spring bolts.  Shorter?

Gary has a point. Maybe someone replaced the stock spring bolts with longer ones. I know I did that to lower mine to even it up with the front end. The front end dropped a little when I put on the American Racing Vector wheels because they had another half inch of negative offset which caused the front end to drop somewhat.

Not positive but I believe the stock rear spring bolts were 6" long.

If memory serves I believe longer bolts are needed to lower the rear ride height.  Shorter bolts, or cranking them tighter, will raise the ride height.  It seems counter-intuitive but it works. 

You are correct but the OP wants to raise his ride height in the rear to match the front.

Which ever way.  My point was that maybe his spring bolts are the problem. 🤷‍♂️