I think you did very well with your purchase! Any manual transmission Cutlasses, 1978 and 1979, are pretty rare. Reliable production numbers are non-existent. I have seen a few numbers poking around the internet but can't verify their accuracy. I've owned 4 manual transmission cars of this vintage and they're a lot of fun. They always drew a huge crowd at the shows I attended because no one had ever seen one with a stick before. Most people had one of those "I had one of these cars when I was younger..." stories but NO one ever had one with a manual transmission. I even had car show "experts" claim that they never made any and that I put it together myself. I showed them the build sheet and dealer invoice and was told those were fake also. I pulled out the owners manual and turned to page 20 to show them the specifics of the transmission(s), and they just walked away. Too bad there are too many know-it-alls that don't know anything.
Your 1978 Calais should be fun to drive with that stick, even with the 2.56 rear end. Does it have limited slip? Only one of mine did not. Don't expect a lot out of that 260 though. It's certainly no powerhouse. The 5-speed will make the car seem quicker than it really is. A positive is that the car was rated at 20 mpg city and 29 mpg highway in 1978 with that combo. Decent mileage even today. Just remember this... even though they're rare... it doesn't mean they're very valuable. I think the $2000 you paid was an excellent price. Enjoy the car!