Hmmmm....Mmmmm...Ahhhh.....fuck I don't know.
Yeah, 6.5" is right for an ironhead neck but that would be including the cups....and your neck alone is at 6.5" right?  You sure there's no step in there like it'd take a race pressed into it?  I don't know bud.....bare neck alone on the 2 early 70's ironheads I've got is right at 5" length.
Sorry I'd check the ID/OD on 'em but they've both got frontends on 'em.