I've been running botton 14mm plugs in my modified 90/6 engine a long time ago. No problems with cracked heads. Heard of but never saw one.
With the 1000ccm and 1070ccm engines I always ran 12mm bottom plugs (heard of cracked heads and didn't want to see mine crack).
Long plugs with an aluminium spacer.
Always NGK BP7ES for the top 14mm plug and NGK D7EA for the bottom plug.
No pinging/fouling or whatsovever problem at CRs of 11.7 for the 1000ccm engines (tested up to 12:1) and 11.2 for the 1070.