Plastic body filler was invented as a quicker and easier way to fill voids than lead. Both work well if used correctly. If you have a gas torch set and the necessary lead tools, using lead is perfectly OK. If you don't, plastic is perfectly OK as well when used as directed. If you need to put much more than 1/4" of filler (either one) in you need to do some more straightening first.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.