Yes, a lot of GT500/350 bits and pieces, separate coolers for transmission engine and diff, along with an updated aero package which increases downforce. There's lots of information out there!
It's a different car, I've had a GT and a Bullitt, and the Mach 1 is head and shoulders above either of them. The only real mod I've done is the H Pipe. Bought mine in 21 and its a keeper!
Yeah mine was full of stones to, check the rear valance that collects them even worse. Good job on getting them to replace it, mine cost me over £300 to buy, and it doesn't fit inside the car to collect it!
Mine looks like that and it's been no where near a race track. Speed bumps or steep drives will do that, and as for oil, the filters is really awkward to change, can get quite messy!