Six pack carb mount bolts

Correct...And no washers either...If originality is'nt a factor then the local hardware store will have what you need...But as stated the correct pieces will have a specific "H" headmarking...

Aren't they also used on non-6 pack applications, such as 2 bl small blacks of the era??

Yes, actually there were many bolts around the engine that had that peticular marking...Headbolts,dist.hold-down,etc...

Head markings are reference to the Highland bolt co. No longer avail from the dealership, but it's obviously avail thru any hardware store if your just after something that works with no regards to originality

5/16" - 18 x 1.25" used from 1968 thru late 1970 or early 1971 on about every V8 , although some 2 bbls used both bolts + studs and nuts combined

If your looking for something close to you that you can pick up fairly quickly, try going to your local Advance Auto parts or Autozone. Ask about the Mr Gasket Carburetor stud kit. It has 4 studs, 4 washers, and 4 nuts in each package. Kit number should be #60. It's not original, but works great. If you are just looking at getting it running and tuning it with the idea of maybe taking it off and on a few times, you might want to go with bolts till you get everything sorted out. The nuts and washers tend to fall out of reach of your fingers and can be a pain when you are working on a hot engine.