As John suggested, the question is how do we fit a decidedly BS&R boxcar into the mission of preserving and continuing the history of the WW&FRy. Up to now, nearly every piece of equipment which has has come to the museum has been adopted into the all time WW&F numbering scheme, and lettered as such. Coach 8, and #10 exemplify this methodology, as they were given the "next in series" number to acknowledge their joining the fleet in the modern era (and never having been WW&F equipment previously), but given them a place in the overall history of the railway. This seems like a good and practical way of meeting our mission while not being limited exclusively to original or replica equipment.
Since our stated mission is to preserve and present the WW&F, it could be somewhat of a distraction from that mission to start representing other railroads simultaneously. And it seems like we've made it a point to differentiate ourselves from the other Maine 2ft museums by our steadfastness in presenting the WW&F exclusively. Seeing a B&SR boxcar on WW&F home rails might give John Q. Public the impression that the two lines interchanged, and we would need to make sure that we adequately explained and interpreted the reality of the situation. Along those lines, photo freights have been mentioned as being spiced up with the inclusion of a B&SR boxcar. I'd suggest that we would be doing ourselves a disservice by including a single interloping boxcar in a string of WW&F equipment, as charter photos are some people's only interaction or experience with us.
I think Stephen's point about interpretation is right on. No matter which way we decide to present the boxcar, we need to make sure that we adequately present it's history. But I would suggest that it is easier to explain the past history of a WW&F numbered car, than explain the lack of interchange between the 2 footers and the boxcar's place at the museum. I think a great way to stay true to our mission, and to 67's history would be to letter it as WW&F and give it a "next in series" number- but create a display of photographs and interpretive text along the lines of the fantastic TCDA display. This could hang on the inside, or outside the boxcar when not in use, and allow visitors to the car barn to see 67's full story.
Just my two cents!