318 with 675/318 or 974/360 heads Cam Recommendations

Boy, you would think that Butte with its extensive mining history would have a couple of machinists. And here is a place in Great Falls that looks like it is a real shop: Machine Shop

Well, the one point not to lose sight of is the balance factor... losing 40-to 80 grams off the piston weight (the exact amount depends on the oversize) is not what I would want in a engine I would rev up. The KB399's is a way to make the piston weight change a lot less. Again, please pick up the phone and call the folks at KB and ask about the actual weights of the .060" oversize 167's and 399's, so this can be computed. Saving that re-balance work cost would be well worthwhile; based on your head milling quote, re-balance work could be $400 or more.

And you DID catch the fact that this SCR of 9.6 with the KB399's is with the 360 heads, right?

As for the 91 octane, what you really care about is not the 9.6 SCR but what is the dynamic compression ratio, DCR. This is effected by the SCR and the cam and cam timing, AND also your altitude. Your altitude drops the DCR by about 8 tenths of a point. so really you are operating with an 'effective' DCR of 7.3 in the Helena area. That ought to be plenty safe for 91 octane.... a stock 318 would be in the upper 6 DCR range, which goes all day long on low octane fuel. I've run iron heads and 8.3 DCR (8.1 effective at 1000' altitude) on the street for almost 100k miles on 93 octane, so you are way better off than that.

And if you jump up to the 262 or Lunati 702 cam, that drops the DCR another 0.2 points. So there are ways to adjust it up or down a bit if you so desire. The mid-7's for effective DCR is a nice range to be in for the street.... torquey but not hard to tune, and you are at or below that, even if you drive to Billing down at around 3000'. (Heck, I bet you could even de-tune it to not detonate at sea level IMHO.) Start easy in the total timing advance at WOT and work it up a bit at a time.