My observation is that guys lucky enough to find an M40 barrel dated 1966-1969 will use them on a clone, even if its worn and provides 3+ MOA performance. These barrels have unique little Remington proof marks and a date code, so historical accuracy is often sought. Not many good M40 barrels exist.
Beginning in 1977, however, all of the subsequent M40A1-A5 barrels were basically unmarked commercial barrel blanks (Hart, HS Precision, Schneider, etc) that the USMC machined and fitted to the receiver. So most clones go with a new commercial barrel and simply have the "RTE-P" or "PWS-P" stamped onto them. Good performance and barrel life is usually sought, and I would think most take-off M40A1-A5 barrels were likely shot-out, or close to shot-out. On the other hand, Schneider barrels on an M40A5 have a very good reputation for lasting a long, long time, up to 10k rds according to Gary Schneider.
It would be a hard decision though if you had the original barrel that matched the paint on the stock. I'd be tempted to build it that way, knowing that accuracy or barrel life might be less than idea. If performance is poor, you can always replace it with a new barrel. If performance is good with the used barrel, then shot it until accuracy drops off , and then replace it. Good luck whatever you do.