With my '51, after removing cab from frame and front and rear angle retainers and all channel suppport bars underneath , I ran a skill saw up the middle of the floor and took it out in two pieces without cutting any metal. To replace it I just put an L shaped butt joint in the replacement oak ply pieces and layed them back in. I'm of course leaving out the details but I feel this is a vialble solution. I think you'll find the one piece too large to slip in under one flange and if you go this route take both side flanges out, (the routed cut-outs that clear the ply panel around the frame humps at the rear axel are very neccessary). One other advantage is you don't have to run down large sheets of wood, two 4x8s do it.
