FWIW, my 360 has 10.6:1 compression using KB107 pistons with the block machined to zero deck. I have magnum heads that have 64cc chamber volume. With a 0.040" head gasket that comes to 10.6:1. The cam is a Comp XE268H and I use a high rise dual plane intake with a 670 vfm vacuum secondary carb. I have 3.55 gears with an OD 4spd.
I get 16 to as high as 18 mph around town and 21 to 22 mpg on the highway. I run the car on 89 octane and have used 87 but it will ping under load at low rpms.
Oh yea the engine makes approx. 370HP based on 1/4 trap speeds.
The heads are the primary reason I can run as much compression and still use 89 octane gas. The cranking pressure is 190 to 200psi so a smaller cam would make more and require higher octane. Also, the OD tranny is responsible for a lot of the decent mileage but you could go with shorter gears.
Another thing to keep in mind is there is about a 10 cent difference between 87 & 89 and 89 & 93 so on a 15 gallon fill you are only talking about $3 between 87 & 93 and 1/2 that between 87 & 89, hardly worth making that a high priority on your build.