You need to orger factory spec. The following gunsmiths do modificationb on LBRS

Alan Westlake (basically anything adding rods/ braces and barrel chopping down to 12")

Roding Armoury ( Cylinders, timing, trigger work ect)

The gunshop cat hill ( you need to ask them)

Valley arms (again cylider work and trigger jobs)

SYSS use to do work. However I think they only do wrist brace to rod conversion now as there were issues with getting Taurus Spares. Either way all the above gunsmiths are more than capable of puttin a rod onto a Buntline and turinging the barrel down from 18" to 12"