I did one 15 or 20 years ago.  At that time Dixie gun works had the breechblocks.  Not centerfire, but rimfire.   

It's not an easy conversion to do because of the weird firing pin arrangement.  I used a captive firing pin, think I used one from an N frame S&W, and modified a rimfire pin into a transfer bar.  

I eventually put the original block back in, sold the rifle and sold the block to Gary Geisler to do a conversion.

Are you certain they won't interchange? Ive seen some information that a center fire block from a Spanish will drop into a swiss perfectly. I have a Spanish peabody and a swiss peabody with orginal us made barrel on the way. I figured i look for a block before trying anything else or swapping them.