Need help with 3” channeled Model A Sedan subfloor

I have a 28 Model A Sedan that has a 5” rear kick up in the frame, chopped 4”. and A 3” channel. Currently there is 38” from the top of the frame kick up to the roof and I plan to use a rear Jeep or low back school bus seat in the rear for the kids, but I am having difficulty envisioning how I am going to build the rear subframe. I know it can be done because I’ve seen pictures of Model A’s with the same setup and rear seats but can’t find any build pictures to help me. Any help would be greatly appreciated.

You got a picture to help us help you. Model A's have been built so many different ways we need to see what we are starting with.

I still have to put a C-notch in the rear of the frame and figure out a different location for my panhard bar but this is what I’m working with right now.

I figure that because of the radius rods, I’m going to need to raise the rear of the floor right behind the front seats all the way back to the rear of the body.

You don't have any exhaust in there yet either. Are they going to run the exhaust out the back, use zoomies , or dump in front of the rear tire... Short of that I would have the rear sitting on the bump stops or remove the shocks and install some tubing in the shocks place to simulate the highest position the diff can travel in the chassis. Then fabricate body sub floor rails and cross bracing channels that fit just above the highest suspension pickup points. You could also make the floor pans lower than the highest pickup points, but you will have to build bulges for your floor pans where the higher pickup points are located. My car isn't channeled but when I back halved it, I fabricated body sub rails that followed the chassis out of 2 x 3 rectangular tube cut in half to make "C" channel and welded another piece of flat stock to the bottom almost like the original rails.https://photos.app.goo.gl/gX3NxhLCJR26RJXp8

How much more jounce travel is left in the rear suspension? I ask now because if there is enough left to need C-notches in the frame the panhard bar is going to contact the top of the housing pretty quickly. You do need to move it but not just for floor clearance. :) Move it down and rearward. Put it behind the housing, keep it straight at nominal ride height to reduce side to side tramp during suspension travel.
As far as the radius rods go just build a tunnel over each one like Keith mentioned. I had to build a couple for my Astro project to the clear the upper link bars. I rolled some 18ga sheet metal and spot welded them into the floor. And remember, the front of the bar is fixed so it will move very little in relationship to the floor so you don't have to go "all the way to the front seat" to have clearance to the bar.
Mark