OK - I think that begins, at least from me, a question back to you. First of all, are those fender well/wheel houses in good enough shape for media blasting? Secondly, have you tried the other and more obvious methods - that being scraping and rotary wire brushing? The reason I'm asking this is to be successful with media blasting of that scale is that you really need a very large compressor a pretty big hose and nozzle. One of those little hand held siphon feed units just wont do the job that I'm guessing you will need on an 80+ year old car. If those area are really crusty and have rust holes, fabbing up replacement wheel wells and floor boards would be my way of handling this. Floorboards are relatively easy, the wheel wells - might have to find another brand, i.e. Ford or Chevy that's close and modify to fit as I have never been able to find elderly MoPar body part.

As always, photos will tell us a better story