Difference in Monte Carlo and SS

Several things really.
SS has a stiffer suspension, black trim, fast ratio steering box, nose assembly, rear spoiler, exhaust and about a 40hp difference in 305's compared to SC's and LS's. Some years the SS's had a higher MPH speedo than others.

I you are just using it for the frame for your car it would depend on the price difference, the SS would have the F41 suspension upgrade with a quick ratio steering box, larger front and rear sway bars and stiffer springs and would still have the 7.5 rear but with a lower gear but if you are using this for your Corvair I would assume you would be replacing or upgrading many of those things anyways since they would be 30+ years old on the donor car so I wouldn't pay too much more for the SS if this is the case.
My 83 Monte CL came loaded and also had the F41 upgrade from the factory so it's not only on the SS.
If the car is clean then the SS parts would most likely go for more money.