Okay so I ran into an issue with the Play Store and ended up reflashing my phone. It turns out I did need the APN fix, but then I still needed to reflash Cyanogen to get it to actually work. So after a little trial and error this is the process that worked best:
1. Wipe for new ROM
2. Install Cyanogen Nightly
3. Boot, get through initial setup, says Sprint (connection seemed fine though, at least for SMS)
4. Flash APN Fix
5. Boot, now says Virgin but wont stay connected (might actually be specific to 3G)
6. Reflash Cyanogen
7. Now I have good signal, confirmed data working on 3g and 4g, and it still says Virgin
Also, although it's probably unrelated and unimportant overall, Google Play is now showing my carrier as Verizon. I'm pretty sure it at least said Sprint before, though, lol.