In the past, using old barn wood, I've been lucky enough to have enough so that after milling the rabbet, all that showed was old barn wood. I was essentially able to use both edges of a long board. I've attached a pic of one that worked out that way (don't hate me for the dasterdly white margin!) This little piece of chestnut will be a different animal. Rather than try in vain to stain/weather the milling that shows, I suspect I'll just leave it as it is, and compliment myself on the lovely contrast I've created (lol).How will you make it look old where you cut it?
Frames from old barn wood have that same problem. The old surface looks great and then where you cut is fresh wood.
No stain will match the old weathering unless you stain the whole thing.