Sorry but it's toast, i have 2 e76 xbox's laying around my shop and they are both victims of a lightning strike. Oddly enough they're both jasper motherboards and have a smd ethernet chip instead of a ethernet chip that uses pins. To fix these you need to replace the burned out ethernet chip with a new one. Though it's more of a pain than you might think because you''ll need a infrared solder rework machine to remove the chip without damaging the motherboard.
You can report the damage to the computer and xbox to your insurance (home owners) and they should be able to cover it. If you claim a large tv that will usually take care of your deductable. The xbox hardly ever gets e76 (out of 600 xbox's i've seen only 2 of them have this error). E74 is an issue with the gpu and microsoft will be able to tell that the gpu is working properly. More than likely they will not warranty your xbox.
Do yourself a favor and buy a new xbox, you're not going to be able to repair this one.
Bachelors A+/MSC - 10 years modding/fixing computers - 8 years modding/fixing xbox's
Owner of West Michigan Computers, LLC