I have well over 400 hp with 15 miles to the gallon. My engine idles smoothly at 600 rpms and my vacuum is at 19 in.
I have an 11:1 400 with Sportsman II heads and a lot of extras.
Here is the kit I used for my bottom end:
http://cgi.ebay.com/ws/eBayISAPI.dll...5&category=317
Build a 400 if you want the fastest streetable SBC.