Welcome, Josh...and that's a great bus for a conversion. We have two 2001 Phantoms, as well. One is sold and the buyer is making payments, so it's in our lot for now. Ours also have Series 50 engines and they've been great. They are great highway cruisers and very comfortable to drive.
You won't be able to get access to any other gearing with that ZF, since it's only a 5-speed...so there's no 6th to unlock, like the MD-3060s. The ZFs are solid transmissions. The only caution is that it will be hard to find service centers, outside of some large cities, if you do have an issue. But that should be a rare instance.
If you contact Gillig they can send you the actual service and parts manual for your bus. It will be specific to your VIN and will show exactly what parts are installed in your bus. It's free and invaluable at the same time.
Here's a screenshot from the Gillig service manual. It shows every wire and the purpose, so you definitely need to get this for your bus. It even shows what crimp tools to use...very detailed.