Hi Gary! I did a fair bit of research before I started and decided that mig is the best way to go. I don't know why the frames were riveted, but I don't think you have to worry about cracking. All of my welding was done with a mig welder. I used 0.035" ER70S6 wire and have an Argon/CO2 mix for a cleaner weld. The only disadvantage is the possibility of warpage which can be controled. Also, as you can see in some of my pics is that I have boxed in the frame for added strength.
Please don't take my word as the gospel, talk to some professional welders/fabricators and see what the best method will be for your particular application.
Here is an on-line article I used as part of my research (of mig welding):
http://www.millerwelds.com/education...rticles86.html
Hope this helps and keep me informed on your progress!
Ross