You can do a 402 out of the 318 if you wanted. IIRC the 390 has more torque then the 402.
Are you doing this yourself or having someone do it?
Here's a decent stroker kit on Ebay.
http://cgi.ebay.com/ebaymotors/318-D...QQcmdZViewItem
Yes, you should be able to bore the heads. Make sure they're checked for cracks and any damage first though.