Even the most experienced member here was at the some point you are at one time. When I did my Cuda (still working on it), I had to do exactly what you are doing. I grew up building these cars and have been a die hard Mopar guy my entire life, but I never installed quarter panels, trunk floors, etc. Back in the day, I built drive trains and took my cars to a body guy when needed. So, I had to learn how to do exactly what you are doing and it took me a very long time to get confident to actually weld it up. Finally, what I did was (as I outlined), I got a pile of Clecos and clamps, I positioned everything and checked, rechecked, and checked again, then I began to slowly spot weld (only a few small spot welds in places where I could drill them out or cut them if need be), rechecked fitment, adjusted as necessary and continued on. Before long, everything was permanently attached and the car looked like a car again; huge feeling of satisfaction.
Here are some tips, you can definitely do this.
- get the trunk floor laid in (Clecos are invaluable here)
- install the trunk extensions (as in clamp/cleco them)
- hand the quarters and clamp them on
- clamp/cleco the quarters in the jambs, fenderwells, ducthman panel, taillight panel
- install (bolt or clamp) the rear valance
- install the trunk lid (you can use the one you have, you just need to confirm overall fitment between the quarters)
Start by looking for problem areas and see if you can determine why it is the way it is. Adjust if necessary. Understand that "adjust" may require cutting and welding. On my car, one quarter basically fell int place whereas the other I had to fight.
Dont be afraid to cut, bend, push, etc. Its just metal and it can be moved and repaired. These cars were not made to a high degree in the first place so don't expect miracles and you will have to accept some level of compromise.
Stand back frequently and see how the body lines look. slow and methodical is the key. Again on my car, I was having a problem getting the taillight panel to line up and I could not quite figure it out until I stood to one side and looked at the taillight panel and realized it wasn't straight, so I used my HF porta power to push it out and then it fit as it good as it could (notice I didn't say perfect or anything like that).
I have installed 2 truck floors (one in my Challenger and one in my Cuda); in both cases I had to trim off the bottom of the trunk extensions.
The real key here is to take your time and don't accept compromise until its as good as you are willing to accept. Also, understand that while body work (aka filler) can hide some sin, it is not the default and your car will only look as good as you make it. That said, there is a limit to how much time/effort you will be willing to put into it and there will come a time when its "as good as it going to get".
Not sure what your welding skill level is, but if you are not experienced at welding sheet metal, you should practice some in order to get the technique and settings right.
Lastly, post pictures of your work and progress here or better yet post what you are planning to do and the members here will chime in and give you some tips/tricks and techniques.
Again, this isn't really that hard it just takes some patience. Believe me when I say that when you are done you will be very impressed and happy with it as it is a major milestone to getting the car back on the road.