I think the factory slantnose cars were all 930 Turbos, so assuming it's a regular 911SC under the skin it was most likely an aftermarket body kit. There's a little history on slantnose cars here:
https://www.911flachbau.com/generations Whether the kit affects the value will depend somewhat on the quality of the kit and how well it was installed, but I think most people are looking for originality these days - a non-Turbo car with the kit is kind of a poser, and is probably considered to be out of style by a lot of people today. Also, since it's been sitting that long it's going to require some reconditioning to the fuel system, brakes, ancient tires, etc. in any case, 911s in general are pretty pricey.
This is from the Hagerty valuation website, of course any valuation guide values should be taken with a grain of salt.
Current Values
#1 Concours $100,000
#2 Excellent $75,000
#3 Good $44,000
#4 Fair $34,000