You can substitute a grade 8 to match. Can't remember the thread pitch. Best option is to shop at the boneyard. Bring the skinny one and use is as your ruler.
That skinny, rotted little b*tch is still available from GM based on what info I have. No guarantees to anything I say here, though.
Both threads: 7/16" x 14
The short bolt. 4-5/8" long. GM part number 15590234. Shows a list price of $3.94 for the bolt from GM.
If you want to pay about double that, you can get one off fleabay today.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=15590234&_sacat=0
The long bolt. 5.25" long. GM part number 9430763.
Don't lose those washers. They're cut to ensure maximum contact. Hard to explain, but if you see them installed, you'll know it due to the curve of the frame.
Those long bolts were NOT cheap even when available from the dealership. I think I paid around $23 EACH with my discount for the long bolts from the dealership. I guess anything over 5" and they'll bend you over and make it feel like 12".