Can't help you on the location of that support bracket, but someone with a long bed truck should be able to measure that for you. If you don't get a response on fender bolt hole locations, I can measure mine on my 3100, which is accessible without the fenders installed yet. They should be the same for a 3600 as for a 3100 referenced from the back of the bed.
An alternative would be to contact
Mar-K. They may be willing to help you out with some dimensions.
