If an object is around 70 #'s, and has it's wieght supported by a bracket, and have a set range of movement. What would be a good size metal to use? Let's say that the bracket is a bit on the not so wide side, (U-shaped - 3 pieces - 2 X 1/12 each piece) would 3/16 and a good weld be enough?

 

i'd say yes but it'd help if you provided more details.

 

50,000 lbs good enough?

Done correctly, the weld will have a strength similar to the base metal or weld filler. Assuming 50 ksi... Figure out the area of weld (for fillet welds, multiply by .707 to get the thinnest part of the fillet). If you have 3 * 2.5" worth of 3/16" wide fillet welds, the weld will hold 3 * 2.5in * .1875in * .707 * 50,000lb/in^2 = 49710 lbs. Weld on the brackets so the welds are in shear (think 2 pieces of paper trying to slide across each other) rather than in tension or bending. If you put funky bending loads on the welds, use poor material, bad welds, etc, the capacity will be much less. Generally all welds are done overkill.