Ok, so it IS a Boss. Well the standard 5.0 takes headers no problem. It is pretty similar to a Mustang, in fact the MAC Mustang shorties will fit with no mods. However the MAC longtubes won't, and the BBK Mustang shorties won't either due to the routing path they chose. BBK does make a 5.0 shorty specifically for the Tbird.
Fact is that the 5.0 is much narrower than the 3.8. The only real problem is the location of the motor mounts (this is minor) and the location of the steering shaft (this is major). I tend to think that is where you will have the most trouble. It's worth a shot.