Originally, I was going to leave the rubber floor in place and cover with carpet, but it was uneven.
When I pealed up the cracked and torn rubber floor, I found some rust spots on the floor so I am glad I did pull it up, luckily only one was over about a 1/2", so they were easy to seal.
Anywhere there was rust, it was treated with rust converter, then if there were holes, they were sealed.
98% of the floor was still in good shape with the original black factory paint showing, so I really got lucky.
Also, most of the contact cement that held the rubber down went with the floor, so I didn't have a lot of prep work to do.
The floor was then sealed with some MIL-Spec 50/50 epoxy primer I had sitting around - that stuff sticks to ANYTHING and does a pretty good job of self leveling.