Teen problems

Pull out an intake bolt.... the early ones were 5/16" and later were 3/8" IIRC. The bolt heads exposed LOOK like they are for 3/8" bolts. Additionally, if these were the early bolt holes at an angle, then the bolt heads would not screw down even with the clamping surfaces on the intake; they look to be seating properly.

I suppose it is conceivable that the holes were re-drilled in the heads and inserts installed for the larger, later bolts. Oops. I see you have figured that out!