[dccarpenter]

That looks pretty similar to my interior and like you I don't believe it is surface rust either. My plan is the same as yours, I'm going to wirebrush off as much of the rust as I can , I'm thinking of using a rust inhibiting/sealing primer prior to the Line-X on both the top and bottom of the cab floor sheet metal. Also, you may not need to remove the axles when you work on the frame, it's sometimes handy to have it rollable to move it around.

Your Blazer looks to be in pretty good condition, are you sure the changes you want to make aren't doable without pulling the whole body off? Might be overkill in this instance, you could probably get it up on high jackstands and replace all the brake lines/bushings/suspension parts from below.

Might be able to find a undercoating place that can pressure wash/wirebrush/paint your frame and underbody for not that much.