Put the truck on stands and take the wheels off . There is a reason that the outer fender and inner fender are two seperate pieces, it is for ease of assembly, your inability to get them on as an assembly shows why it is not stupid. The bolt lenght is not important. I have replaced all of the bolts on my '90 with the same length. The 25 mm length wiil work fine for all locations. Fit the outer fender first.
There are one bolt that goes through the fender into the core support front of the outer fender on top at the very front end, two more when you open the doors on on each hinge facing forward, one from the bottom facing up behind the rear edge of the wheel opening, one facing down under the hood mounting pivot and depending the year two or three on each side of the core support. one goes through the fender into the core support right above where the bumper is. The other bolts come from the core support in to the fender. There are shims needed at the bolts in the hinges and under the hood pivot locations. once that is aligned THEN you worry about the inner fender.
