Replacing the front body mounts requires paying attention to a couple things. If you replace the floorboards be sure your floorboards are level so there is an even 1/4-inch gap between the mount and frame.
THIS is what you want to avoid.
I would suggest getting your frame up on jackstands and shimming under the stands until the frame is level left-to-right. Then when you install your floorboards be sure they are level and you are good.
Also, temporarily fasten everything with #8 sheetmetal screws. It's a lot easier to unscrew something than it is to unweld something.
To avoid removing too much sheetmetal I used the half floorboard patches and did one side at a time. It's not easy getting them in in halves and considering the amount of rot I had it would be a real chore wrestling a one-piece patch in there. If your cowls and sills are solid it might be easier.
Good luck.