I still don't see why it is a bad frame to start with. Sure old deraillers/ tensioners may be kludgy, but they get the job done. I've built a few beater/winter bikes w/ vertical drop-outs and an old clamp on rear entry level SIS derailler. One was a late 90's GT Talera 4130 cro-mo frame.

As stated earlier, horizontal drops are key. My Giant has them and it made set up a breeze.

On my Trek SS I used a White Industries ENO eccentric hub. At the the time it was $179. Add a $70 rim and DT double butted spokes and it neared $300 after labor. Expensive yes, but I expect the wheel to outlast multiple frames. Something to consider if your GT is a starting point and you plan on having a SS built up in the future, the wheel is already done.

As I see it you have a few options, some pricey some not. Regardless the sooner you set up an SS the sooner you can experience great beater bike w/ less drivetrain maintence.

Craig