2001 Ford Truck problems with power

I have had 2 transmitions put in my f-350 truck in the last few months. The first one lasted about 300 miles before it started on fire and all of the fluid dumped out between the engin and trans. The current trans dosn, t have the power or fuel economy as my original one. (From 15-16 mpg to 7-8 mpg) I had anouther macanic check out my truck and found a bent flywheel and a leak in the rear main seal. Could the heat from the first trans cause the flywheel to bend and rear main seal to start leaking and could this cause my milage to drop. Or would the bent flywheel be caused from the ford dealer who installed the trans. What else could cause a flywheel to ben or warp. Thank you Jarett