Get a dial timing light. They are a wonderful tool.
Set the total (int & mech) to 28*-30*. Whatever int that gives you is what you are stuck with, unless you do what Slantzilla advises, weld up the slots to reduce mech and allowing for more int, you'll be glad you did. The alternative to welding the slots, get the advance plate from a dist that is marked with a '9R', this will allow you to run about 12* int and 18* mech, the '9R' is what a few slant sixes came with.