It will probably fit... However, It will most likely be very close to the hood which is fine as long as you have some good motor mounts and you might want to put a little of the self adhesive reflective high temp wrap on the bottom side of the hood to protect the paint... If it touches the hood a small aftermarket hood scoop could be installed to fix that...
The down pipe that you have will not fit the GT-42 so it will need to be modified or make a new one out of 4" If you need any help making the down pipe or exhaust let me know...
Dave