Update:
No luck finding a gunsmith who could do the work, so did a bunch of research and took matters into my own hands.
I found a drill bit that was pretty close and reamed out the chambers myself, now it chambers .454 Casull cases nicely. The only problem is, the .454 I bought were a bit too long, so I seated the bullet deeper in the case by gently squishing it in a vice.
Can't wait to try it out! Stay tuned for a range report.