Will definitely need dish pistons in order to get by with 91 octane.

The cheapest dish pistons will probably be having the machine shop machine a dish in the SP L2262F pistons.

Next cheapest will probably be Icon brand dish pistons, bought from CNC-Motorsports. Their price is $500 + shipping.

The next best price, and my choice, are the Auto Tec brand pistons. I think they charge around $150 to cut the dish. But again, you can get a custom pin height, in order to reduce the amount needed to be machined off the block decks. Will only need enuff cut to square up the decks.Then order the pin height you need to achieve zero or near zero deck height.

With zero deck height, you can use the thicker & cheaper 8518PT head gaskets, and have only a .045 quench distance. Many say .035-.040 is better. The more expensive Fel Pro 1016 gaskets are only .039 thick. This will give a slightly smaller quench height, and raise the CR slightly.

I've read that the 670 heads are more detonation prone than the '68-up open chambered heads. So, it might be a good idea to build with no more than 9:1 CR, I think the chambers can be opened up some. But that adds to the cost of building the heads.

If you get the compression up between 9:1 & 9.5:1, you can use a cam with more adv duration.

The smallest cam I'd consider is a Summit 2801. A Summit 2802 will give you more idle lope & upper rpm power. The Lunati 10510312 & Crower 60916 fit between the 2801 & 2802. The biggest cam I'd consider is a Crower 60243. A Crane 283951 fits between the 2802 & 60243. Just barely above the 60243 is the Howard's 410051-14.

If you have the chambers opened up, you can probably use the Voodoo 262.

Some still prefer the 068 cam. But that cam has a smooth idle and just barely over .400 lift.