Ya know, I've been kicking this combo around for about 2 hours now, and here is the conclusion I have come to after re-reading the OP's mission statement several times.
Bore the block 0.030" and fit pistons for 0.0015" to 0.0002" piston to wall clearance. Gap top ring at 0.026", gap second ring at 0.017".
Pistons are 19.5 cc's in a step-dish configuration and will produce a static compression ratio of 9.5:1 when used with a 58cc combustion chamber. This motor will make all the power you need at 9.5 and will be more resistant to detonation. I've been wrenching for 65 years and this is the motor I'd build for myself.
KB303KTM-030 kit includes pistons and rings. I believe Spiro-locks are included in the event that you want to full-float the pins.
Cut the block decks to 9.480" block deck height and use Fel-Pro 1011-1 composition gaskets. Piston deck height will be zero and Squish-quench will be 0.041". (Crankshaft radius is 1.750", rod length is 5.956", piston compression height is 1.774". Added together, this makes a STACK of 9.480")
Flo-Tek 180 aluminum heads
Scorpion roller rockers, 1.6 ratio, 3/8" studs. Will free-up 20+ hp due to reduction of friction and also will lower oil operating temperature.
Howards CL221275-12 retrofit hydraulic roller cam and lifters kit
270/278 advertised duration
217/225 0.050" duration
0.528"/0.533" valve lift
1200-5000 operating range. Will rev higher, but motor will be on the downside of the cam. Will cruise well at 1700 and higher. Should make great mileage at 1700/1800.
Flo-Tek says their springs are good to 0.550" lift, but check for coil bind at full lift. Use feeler gauge blades between the coils.
600/650/700/750 carburetor on Typhoon intake, 14" X 4" air filter element so the motor can breathe. Smaller primaries will add crispness to the motor. Hook up PCV and run distributor vacuum cannister off full manifold vacuum, NOT PORTED VACUUM. Put 12 degrees initial in at the crank and limit full initial/centrifugal to no more than 34. I have used only Carter AFB's and Edelbrock Performers since the late 60's and have found them to perform flawlessly when properly tuned. Edelbrock publishes a bulletproof tuning manual. Limit fuel pressure at the carburetor inlet to no more than 5 psi.
1 1/2" or 1 5/8" full-length headers using 3/8" thickness flanges. Thinner flanges will warp up from the heat of operation and spit out the gaskets. X-pipe or H-pipe immediately after collectors. Run pipes to rear bumper through mufflers of your choice to prevent car sounding like a Ricky Racer ride and having all the old guys laugh at you.
When someone asks you what the hp is, tell 'em THREE-SEVENTY. More than that would skin the gears off the shafts in the box anyway.