Exhaust System on '67 barracuda
The problem is not that they won't fit but more how they would need to fit. The car has a recess for mufflers just in front of the rear differential to keep ground clearance.
To install a leg pipe it would need to change a lot of direction like a backward broken S. At that point I do not see it as being efficient. The other option is to use an AAR/T/A muffler which enter/exits on the front and turns back again probably not the best and does Flowmaster make a muffler to do this?
T/A exhaust example:
http://www.accurateltd.com/AARTA-Complete-Exhaust-System_p_187.html#
Making regular tail pipe(s) seems more logical and I imagine no more/less work or material for a better flowing result.