Need a plan - component selection for restoring 68 340 that runs on 91 oct

Well, your fuel limitations do not deny you power.

With the stock engine, and all the stock components, the only thing that I can see to be changed is the compression ratio if that year engine has a high enough ratio not to work with 91 octane.

If the engine is all stock and running, then I would use a thicker head gasket to drop the ratio a tad. You’ll need to do some investigation and math to get to a pump gas friendly ratio. A 9.0-1 should do it.

You’ll need to measure how far down in the cylinder the piston is exactly. Then the valve reliefs must also be cc’d, I’d there is any. Bore width, cylinder head cc amount, and then head gasket width and thickness. (The one that is currently being used and the one you want to use. You can then go to “Wallace racing” (Google that name) and find the compression ratio calculator.

Cometic can custom size your head gaskets and they have many in the shelf in various thickness from a low (& may be lower) of .030 up to .100 (and maybe thicker!.)