There will be two things working against you.
The years on your truck have caused it to move an flex so it will not be perfect for even quality used parts.
And, repo parts just don't have the quality that you want to expect (it is what it is).
Bottom line get a 2x4 and pry or hit where you need to.
I got my inner fenders from Classic Parts. Overall they were pretty good but I still I had to grind metal and pry where necessary.
I also did not trust the metal from China so I had mine sandblasted and painted them with POR15. Then primer and paint.
Stay after it and you should be good to go.