Not every 340 is 1500 bucks. You can buy a block for 500-600 bucks. Buy the 318 for a hundred if it has a steel crank and use that crank and rods. Cast iron heads are cheap and getting cheaper. You’d be in the 340 less than 1500.
As for bore size and horsepower, the late (RIP) Harold Bettes said it best. If I can find his book I’ll take a picture of it and post it in this thread. The bigger the bore the more power you can make. And it’s not from air flow. A .060 change in bore size does almost nothing for flow, unless the intake valve is shrouded by a ridiculous amount.