I too believe I have a 69 Cat, Bonnie, GTO motor in my bird (YD Block). I need to call RA Resto and to confirm the longbranch fits ok.
Maybe headers render more horsies, but if I follow everyone correctly, the longbranch seems to solve alot of problems..
I'll buy them for the dyno testing, but install my stock manifolds so I can drive it to the muffler guy. Then pay to have my exhaust done again, thereby keeping my record of doing everything twice intact.