I'm not much help for your particular question either, but only one of mine was splined, the others were standard high grade bolts.
IIRC
Frame ~1/4"
Bracket ~ 3/8"
nut `5/8"
total thread needed ~ 1 1/4 - 1 1/2 or so. Mine were probably ~2" but too long. one was longer than the rest (the splined one) and it made it easier to install that way.
It was last summer that I did this last so I am fuzzy on it.