I was able to use the original floor underlayment. You could say it fits nice as it's had 59 years to get comfy in there
Then the first pass on the front rugs. Using the originals as a template, I made some initial cuts and began to fit them in. This morning, I started to steam the rugs in order to get them to relax a little more. A bit more trimming will be in order, but I can sneak up on that.