Timing 98 GMC 350 5.7

Ok have a 98 GMC 350 5.7 1/2 ton the thing runs good, but most every time I have to pass someone the engine clatters like the timing is either to advanced or just the 89 octane crap. I have a really good scanner that will read live readings and will read timing and I also have a high dollar timing light, do I have to pull something like the vacuum line like on the old cars to time it? or just turn the distributor and watch the timing on my scanner? basically what would be the procedure on this truck? I know some will say you don't time them, but I think your wrong as for as what I have been told the timing can get out so bad the computer can't compensate.
Thanks
Mr. Bill

This should really be a sticky but here is the procedure: http://www.automotiveforums.com/vbulletin/showthread.php?t=350633

//2000CAYukon

This should really be a sticky but here is the procedure: http://www.automotiveforums.com/vbulletin/showthread.php?t=350633

//2000CAYukon


Thanks Brother and your probably right this should be a sticky.