Good news is that everything forward is brand new.
Unfortunately, I hate to break it to you, that is both good and bad news. After market sheet metal is notorious for not fitting properly. Many bolters have drove themselves near crazy trying to get the gaps and alignment to factory specs. A good example is a fellow bolter “Fox”, that bought an entire new bed for his build and ran into issues when the tailgate didn’t match the rear right bedside. Here is a link to that post. https://www.stovebolt.com/ubbthread...ndpa-s-1970-c10-rebuild.html#Post1446340
The front end is another hurdle you sill have to tackle, but you have to get your cab together first. Don’t look that far forward, as you will only get discouraged. Take the job at hand and research. That is what this forum is for, so you don’t make the mistakes that we bolters already have.
If it was my project, my first thought would be to reinstall the old floor temporarily with tacks, then brace the doors.
If that isn’t possible, I would hang the doors back on (don’t fix them yet, use them as is because you will use them to locate the lower hinge and fixing them now will only compound the issue), even if you have to tack weld an angle iron across the skin of the doors to the sheet metal on the body to locate the door and establish the gaps then use the lower door hinge to locate the “A” pillar and tack weld it in place. Don’t weld it in completely until you fit the floor and the other associated panels. Next brace the door opening both horizontally and diagonally, then remove the door. This way you will know that the lower hinge and pillar is in the right place.
Good luck and don’t be shy about asking questions.