670's were GTO-only heads in 1967. The flow pretty well, but they're closed chamber heads and usually need quite a bit of timing. 400 heads on a 350 can work, but you need to make sure there's no interference between the valves and the block. Even 400's had chamfers at the tops of the bores for valve clearance. My handy-dandy compression ratio tool tells me that 670's on a +0.030 Pontiac 350 that's in all other respects "factory nominal" (always measure everything first!!!) will put you in the neighborhood of 9.4:1 static compression. With the right cam, it should live ok on 93 octane as long as your cooling system and fuel system/mixture are spot on. One concern I have is that the smaller 350 bore with the "usual" .020 Pontiac deck clearance is going to have some "prime" hot-spot, detonation-inducing real estate in the chambers due to the top edge of the cylinder bore. The closed chamber 670's may help with that some if the flat part of the head decks completely cover the cylinder bore edges.
Also, forget your 20+ years working with chevys. Very little of that detailed knowledge applies. Pontiacs make torque and they make it down low. You don't NEED to spin them fast to get things moving, so you're not going to need a lot of rear gear. My 69 GTO weighs 4000 pounds or so with me in it, has 3.50 rear gears, and has run a best so far of 11.86/113 at the track. Engine dyno says it's north of 500 lb.ft. by 2800 rpm.
There are two good reference books out the for you that will help:
Both have the same title: "How to Build Max-Performance Pontiac V8's"
One is by Jim Hand and has been "the standard" for years, the other is more recent and is by Rocky Rotella. I recommend you get both.