I've got what (if memory serves) is a clutch equalizer (Z) bar for a 63 R code Galaxy. But the same bar for a 390 is very similar. This bar is massive. Maybe the 390 equivalent is a little less robust?
Are there markings or anything to tell them apart? I drag raced an R code in '71 after putting a C-6 in it, so this probably came out of it. I don't know why I would have the 390 version, but I want to be absolutely sure.