I have a Mig welded joint that needs to be bent back to shape. It is a different kind of joint is and is almost impossible to tack and get straight before welding. If it is possible, I know I can get it to shape by heating it in the forge and bending it; then maybe rewelding it; is this possible?

It will probably work. I often MIG weld joints then hammer them flat after heating and I have never had one separate but I don't know what your project looks like. Make sure you get the metal close to welding heat before reworking it.

Like HWooldridge said, get it real hot, and don't work it cold. Make youself stop while its still hot and reheat it.

depending on what the joint material is ( bar, rod, square, hollow - read tube or square ). Acetelyne and a gas saver is great in this situation. As has been noted, you need it pretty hot. Acetelyne may scale a bit and this can make for a cheesy joint if you not careful. Pile some wire on and smooth it with torch THEN forge. Yes, a hot file works well. you MAY kick the fire up and back off on the wire a bit, then weld the joint while hot from the forge ( read lay wire like caulking ). I frequently use migged joints in production and they disappear nicely with the torch. I have not had any returns issues and I am not scared of the procedure ( meaning I stand by the joints ). YMMV