Yep, Tech II. Otherwise you're shooting in the dark. It will tell you the exact fault. You can then clear it. If it comes back, you have a physical problem. I'm suspicious of your seat belt as that's the only SRS component you changed and you DID disconnect the battery, but it's also possible you disturbed something. I'm assuming you remembered to hook up the little connector right on the seat belt(s) after install (not that I ever forgot one, nope!).
Like Jeremy said, it will eventually reset. Eventually. Assuming there are no faults and it was a transient issue. BUT, you could end up waiting a month and getting no joy if there's still a fault. Also, I tried a zillion starts/shut-offs and it didn't seem to help. I believe it's drive cycles needed to reset, not just start/off cycles
FYI - there is one possible shortcut for you: If the donor car had no SRS lights on, and if the part #'s match on the SRS units in the two cars, you can swap the donor SRS computer into the car with the light on. If there are no faults, the light will be off. It's a one shot chance because once it sets, you're back to needing a Tech II. So disconnect the battery, re-check your connections, then swap the SRS computers. You might be all set. If the light comes back on then there's a physical issue.