I am going to put a P90 on my L28. It has 1mm over flat top pistons and a mild Snyder cam. How much should I shave off the head and do I need to raise the cam bearing blocks to account for the head being shaved.

Here's a couple of pages with decent ideas on this:

http://www.geocities.com/zgarage2001/p90.html

http://www.geocities.com/zgarage2001/head.html