Mouse, do you have a 4 1/2" angle grinder? If so, take a look at a pack of flap discs from Benchmark. They take metal down much faster while providing a better surface finish than a pneumatic die grinder. Only time I pickup my die grinder now is if I have to get into a really hard to reach space.
Benchmark Abrasives
I have a cordless 4 1/2 angle grinder and cordless die grinder. I cant really get the angle one on the inside and was using a grinding disk to mostly cut down the outside, then used the burr in the die to get the low spots. Those are some nice flap disks tho, been using hard ware store ones to knock rust off the frame.