Here's the links to Bruce's write-up on ignition timing.. the first one was post #712 of his build page and you'd also want to read his follow-up which was shortly thereafter..
Many thanks to Bruce for all he has helped with his efforts!
http://67-72chevytrucks.com/vboard/s...&postcount=712
http://67-72chevytrucks.com/vboard/s...318544&page=29