Have some more info on the parts.
Dimensions for GM#8869561:
Head DIA: 3/4"
Thread DIA: 7/16"
Threads/Inch: 20
Overall Length: 1 7/16"
Studbase to Bottom: 9/16"
Studbase to Top: 11/16"
Dimensions for GM#491253
Head DIA: 7/8"
Thread DIA: 1/2"
Threads/Inch: 12
Overall Length: 2 3/16"
Studbase to Bottom: 11/16"
Studbase to Top: 1 9/16"