Mike,
I’m fairly confident that all of the intake manifold bolts are 3/8-16 x 1 1/4” including the bolts that secure the lift bracket. I’m also fairly confident that the two bolts that secure the lift bracket are not shouldered versus all of the other intake bolts which are shouldered. From previous threads on this subject I thought perhaps that the two bolts that secure the left bracket were slightly longer (3/8-16 x 1 3/8”) to compensate for the material thickness of the bracket. In fact, I even purchased a set of bolts (
https://www.paragoncorvette.com/p-355394-intake-manifold-attaching-bolt-kit.aspx) from Paragon Corvette which supposedly had two longer non-shouldered bolts. When the bolts arrived, all of the bolts were the same length! A quick chat with Paragon indeed confirmed that all of the bolts were the same length and that their website is wrong! I’ve also come to learn that 3/8-16 x 1 3/8” is an extremely unusual grade 5 bolt size. I’ve yet to find anyone that sells this bolt in that size.
So at the end of the day I’ve decided to use the non-shouldered 3/8-16 x 1 1/4” bolt that came with the Paragon kit. It would, for sure, be interesting to see what the length of the bolt is on your ‘67. My guess is that it’ll be the 3/8-16 x 1 1/4” length bolt.
Thanks for all your help!
PS: The rear engine lift bracket bolt is sorted out since this was a commonly used bolt.