general problems of the 2005 Hyundai Sonata

My car's engine cradle, cross member or subframe is rusted out and the oem replacement for my VIN number won't fit it correctly I'm getting really upset because this could be dangerous for me and my two young kids. I've already paid the mechanic $600 for the replacement part when it really only costs $450 at the most and the two he has ordered before have both been either too big and it says that it is the correct replacement part. What is going on?!!! if something isn't fixed soon and something happens to me or my kids there is going to be a lawsuit.

Ever time I press the brakes hard the engine turns of completely.

While driving on the freeway the car started accelerating on its own. Tried pumping the gas pedal to release it, car still accelerated. Brakes did little good. Was able to down shift into first gear, then into neutral and utilizing the emergency brake was able to get the car slowed down enough to cut the engine. Some problem happened immediately when I turned the car back on trying to get it off the freeway off ramp. Was able to stop safely in a parking lot without hitting anything. Had the car towed to the dealership, where they said it was a problem with the throttle.

2005 Hyundai Sonata, engine in this car was meant to be in the elantra which is a much smaller and lighter car, but when put 4 cylinder engine in a Sonata which is a heaver and larger car, even though engine operates to the spec, other problems are evident which the dealer and the manufacturer failed to tell the consumer, now this car idles so bad it is worse than a truck when car is in drive and stopped at light etc.