I have to agree, I don't think anybody here was trying to intimidate the OP. When I first read this thread, several members had already posted and I felt I didn't have anything to add that would have made a difference and agreed with what was posted. I read what the Op said very carefully and because of what the OP said, I'm sure that this is what prompted the responses that he got. I respect the Op for stating how he felt about what he was doing, how he wanted things to be done right, the fact that he would obsess if what he had done so far, or what he may do in the future wouldn't be done correctly. Giving advice on how to do things is difficult, wording it so that whomever your trying to give advice to completely understands often is next to impossible, no matter how much command you may have of the English language, what you write and what the person reading what you wrote, may be translated in a completely different light.
Telling somebody how to do things is like school...showing people how to do things is more like real life and in my opinion goes a lot further in teaching. Sometimes trying to pass on knowledge can be frustrating, on the internet, anyone who has done this before for a living has the benefit of is, writing and posting pictures. The hands on of stopping someone when you see that they are making an error, to show them a better way or to just say, stop now or your going to over sand that panel can't be done in the environment that we have here. Very often, what some people may feel are "pro's" hear, is coming after the fact when the person asking how, has done it wrong, maybe it wasn't explained in the terms that the person asking could fully understand. Maybe the person asking how, thought he understood but didn't. When I hear someone who has started a thread say, "I ask these questions only because I will obsess over it unless I know it is Good", and the advice given, I'm sure was given with the best intentions for the OP and have reread every post, not one member in my mind crossed a line and tried to intimidate the OP.
What I'm more happy about is that not one post said, "this is easy, all you have to do is"...those are the posts that do more damage than people telling someone that they have a big job in front of them and give alternative suggestions. Those box sides would be tough, even for a seasoned "pro" and I know many seasoned pro's that either wouldn't touch them or if they did wouldn't get them straight...yet they have a piece of paper that says they are pro's. Every person that offered advice knew how tough those box sides are and reading your post, your the only one that has made it sound not nearly as difficult as it is.
Comments like "Anybody can learn body work" and "Getting sheet metal straight ain't that hard" or "Nothing too difficult to accomplish here". I had a few more but I'm sure that my point is made. You may feel that these quotes have been taken out of context but, how do you know how a person will interpret what you wrote. Everyone of the three quotes is wrong. Not just anybody can do or learn body work, I've met numerous good people that just can't do it, no matter how hard they tried. Getting large 6 foot surfaces of sheet metal straight is difficult, you need patience, you need passion, you need experience and know what you are doing.
My hats off to the professionals that took the time to give the OP their opinions and offer advice so that the job would turn out best for the person concerned. My hats also off to the OP, his comment about obsessing tells me that he does have the passion, I would hate for an individual such as this, who obviously cars about this vehicle go ahead, grab a 2 X 4, prep his truck and not be happy with the results. It may demoralize an individual to the point that they might never want to attempt anything like this again.
Only wishing the best for the OP.
Ray