A couple of things. From what I have read, frames, especially truck frames, are made out of different materials, depending on manufacturer, model, year, etc. Some of the materials are "NO WELD EVER", some are OK to weld on, if it is done properly. A metallurgist could tell you what type yours is if you know what the original material was, and what, if any, treatments it received during manufacture.
Using bolts is almost always safe, if you use the right bolt. Remember, though, that a channel section has a neutral axis at it's center. Above the neutral axis, compression forces increase, and below the neutral axis, tension forces increase. Naturally, the closer you stay to the neutral axis, the less stress/strain on the hole. Remember that any hole in the frame is going to produce a 'stress riser'. That's why stress cracks tend to radiate from holes. Peening holes can reduce the stresses generated significantly. (Put the ball of a ball peen hammer in the hole and hit the flat head with a brass hammer, or a dead blow hammer. Wear safety glasses anyway.)
Basically, I would suggest that everyone read Carroll Smith's book 'Nuts, Bolts, Fasteners and Plumbing Handbook' ISBN 0-87938-406-9. I think most of you will stop worrying about whether a bolt is listed as a Grade 8 or a Grade 5. It seems that, in many cases, the Grade 5 may be more suitable than the Grade 8. Naturally, AN/MS is really the only way to go for critical components, once you get used to the sizing system.
Good hardware around here is hard to find. There is an O'Reilly, a Do-It Center and a Wal-Mart withing cruising range, but they usually don't have exactly what I need. I've been having good luck in both quality and price from
McMasters & Carr For instance, I needed an 7/16" bolt eight inches long, 14 tpi for a generator mount for one of my trucks. For $20, not counting S&H, I got the bolt in Grade 8(?), 25 heavy duty nuts to fit, 100 flat washers and 25 lock washers. I've tried to find that bolt at LMC, etc. No luck.
