Think of it this way, have you ever seen a
truck frame (the frame itself) that was bolted together? Not the attachments to the frame, but the actual frame itself?
Therefore, from a pure strength and mechanical engineering viewpoint it is better to weld it in... but:
If you plan to use your truck, as a truck, hauling and towing, then you want a notch designed for towing and hauling and it should be welded to the frame.
If you plan to use your truck purely as a means of transportation, no hauling or towing, then a bolt in kit is sufficient.
Some of the simple c-notch kits are scary! Go with a reputable kit from a reputable company: Bell Tech, CPP and Porterbuilt all build a quality notch kit.
Here is a link to other posts that will no doubt provide you with hours of reading:
http://67-72chevytrucks.com/vboard/s...=1957427&pp=25
Good luck and be sure to buy a good quality kit. This is one area that good engineering truly matters.