Welding doesn't hurt the metal because it was very mild steel. However, welds don't flex. Rivets and flange bolts do. These are the type flange bolts we use.
http://www.imperialsupplies.com/item/0191980?service=/browse/

I have been making frame modifications on heavy trucks for many years and this all I use. I use 3/4 and 5/8, but 3/8 or 1/2 is big enough for a 2 ton.