A 318 is nothing but a 340 with 22 less ci by bore, what you're wanting to accomplish while keeping a reasonable budget is quite attainable, the Super Stock 318 heads only flow about 240 cfm @ 28".

I'd send a PM to member W5DART66 to see if he'd be interested in doing your heads, along with recommending a cam and other parts. He has experience making big steam with the 318 OE based block platform.