Most Fentons are off shore copies. Some are not done as well as others, but no way to tell which you will get.
You will need to replace the the locating pins (first and last bolt) with studs (if GMC has them) and you will have to play games to get hold down washers to come out even (grinding or adding spacer blocks, etc.)
Fenton design was to allow the owner to bolt up the original pipe to one header and just make 1 new exhaust pipe.
