It is my belief that:
The 68 short spoiler #3916633 GM spoiler 40-2259-GB uses; Three lengths of Studs,
Double ended, 1/4-20unc x X (plus 1/4")" x 1/4-12, 1/4-20unc machine screw thread end Dog Point, 1/4-12 wood screw thread, blunt.
Qty: Y
Length: X (Exposed Machine Thread- including dog point)
Root Length: Equal __1/2"__ on ALL (Root is the wood screw thread portion- Blunt)
The Stud at emblem position is the Med, The studs at the deck lid edge are the Shorts, the remainder of the studs are Longs;
Shorts: Y = 4 each, X = 1/2"
Meds: 1 each, X = 1"
Longs: 3 each, X = 1-1/4"
Dog points machined to thread root diameter and length appx equal to or less than 1/4"
The nice thing is the hardware aftermarket is fully supported via McMaster-Carr 1/4-20 x 3" Hanger Bolt Zinc Plated can be sourced and machined via a lathe or other means to replicate damaged or missing studs. 1 package contains 25 each, HXA04096MC 90207A310 Cut down and turned as needed to replicate replacements as a DIY
69's seem to be a different animal (Not Dog point end)
Both My 68s have owner modified early in life with Short Spoiler of GM fabrication & are both equipped as described above.
My 69Z11 have studs like Gary's 69Z.
Jim