derek uk wrote--"You will need to up the compression ratio a bit to give it some help but not too much if you want to keep using regular gas. It sounds as if you have a fairly early car so could have the single outlet exhaust manifold. Changing this to the later twin outlet type will give you an immediate 2-3 bhp even with an A cam , maybe a little more with a K. Does mean you'll at least have to fit a new twin pipe front exhaust section."
The OP must be careful about any compression ratio increase. AFAIK - at least for the US market - the motor should have a twin outlet manifold/front pipe - and if it does it will already have a compression ratio of 10:1 and rated at 115hp (lowered to 9.3, Stromberg carbs and 118hp for the B20 in 1969). Measuring the deck height or checking combustion chamber volume should be done before any milling takes place.
BTW--in a Sports Car Graphic series on souping up the B18 (1963) they dynoed a 12hp gain going to a twin outlet header! -- Dave