Hey there ohten! I ended up just putting the spigot.jar file in the ClassicFinal folder out of impatience, and that seems to have worked fine. I also had the initial issue of not being able to use any plugins, but I just opened the console and typed OP [my username], and it gave me all the necessary permissions. It seems some weird things are restricted to OP that never used to be, such as homes and warps and whatnot.
Assuming the plugins are actually present in your install, if you OP yourself with the console it should all work fine.
( o つ o )