Technical Replacing whole floorboard

Im replacing the floorboard on my 52. Using one from a 53 Special parts car wich should work fine even tho a more door. I need to replace it with inner rockers as the ones on my 52 are rusted and seems like the best method both time and labor since im going with factory spot welds.
I asked my buddy in Sweden about it and he sugested a X brace going from packagee tray to A post and bracing on the doors as well. Its all dandy and stuff but how will I drop in the replacement onced the X brace is in?? I have never done a complete floor nor seen it done why I am here today. Nothing like replacing a 1/4 panel thats for sure lol. Car sits on frame ovet jack stands. All feedback is welcomed safety first

 

The rocker box is a key structural component. Make sure you have adequate bracing before it is removed, or if the rocker box has already evaporated due to rust, make sure the body is positioned correctly, doors fit, etc, before welding in the braces. You don't want to weld in the new floor and rockers and then realize your doors don't fit after the fact.

X braces between door jambs, the package tray, and dash are all good. I just did the firewall and part of the floor in my '51 F-1 last year. You can see the brace I fabbed up in the image below.

 

Is the "new" floor pan going to fit inside of the car, even without the X for the body bracing?

I'll be watching. The only way I have ever heard of replacing the entire floor was to brace the body, loosen the old floor pan from the body by cutting the spot welds on the inner rocker and lifting the braced body off the old floor pan. Then the old floor pan was removed from the frame and discarded.
The process was then to clean up the rockers so they were ready to be welded back together. Then to install the new pan onto the frame (with new body mounts), lower the body onto the floor pan, and weld it up. Then remove the body bracing.

You obviously can't do that, at this point.
I'm sure there are probably U tube floor pan installation jobs posted.

Were it me (I have done a few floors over my 30 years of welding shop ownership), I would probably clean up both the body rockers and the floor pan rockers to shiny metal so welding can be done. Hopefully, you and some help can carefully slide the "new" floor pan inside of the car without destroying the door alignment. Once the floor pan is inside of the car, door alignment (and the body lines around the doors) and the door function are going to be critical. If by some chance you are still good, brace the body as good as you can. Then, hopefully you will be able to lower the pan into position without screwing up the rocker alignment. I would set the "new" floor pan onto new body mounts, you want to get it down into position with minimum force. At this point, I would probably rather trim a bit of clearance (as little as possible, sliding into place is much better) rather then force the pan into position. Once the pan is in position and bolted down (on new rubber mounts), I would spend a lot of time being sure the doors are still aligned and function correctly. This is the time to correct any door alignment issues. If the bracing needs to be cut, do so very carefully, then re-brace as needed, you want it pretty solid, you will be leaning over the top of those braces.

With the door alignment and function correct, its time to attach the floor pan to the rockers. As with any sheet metal welding, tack weld the contacting places, and spread those tack welds around the entire floor pan. Don't do more then 2-3 tacks within a foot of each other until you have the entire pan welded that much. Once the first round of tack welds have been done (4" - 6" apart) all around the floor pan, check the door alignment again, door alignment is critical, and this is the best time to adjust it if need be. Make a round of spot welding between the existing welds (skip around). Make as many rounds of spot welding you need until your welds are equal to the original spot welds on the rockers.. Fill in any clearance t******* you did earlier. Sean seal the seams after the welding has been cleaned up.
Good luck.