I am in the process of doing the same project you are, my friend. Mine was built in 1940. I would suggest NO SHORT CUTS! Go ahead and take out that radiator, old tile floor, plaster wall and everything and start fresh. When I did as much, I discovered studs that were non-existant from about knee level down! Had I done my remodel over those bad studs, the whole thing would have crumbled eventually. I also ended up replacing all the wiring and lots of the plumbing including new copper supply lines, p-trap under the tub and new toilet flange. Do you really want to put drywall around a tub? Your call but I would suggest backerboard and tile around a tub. Whatever the case, you need some "nailers" on that brick wall (you mean concrete block wall, right?). My tub is against an outside block wall too, and I installed "nailer" boards. Just strips of 1x2 or 1x3 on 16 inch center using construction adhesive and tapcons. My backerboard went right on no problem. Just make sure you coordinate the length of your screws and width of your "nailers!" Well, that's my 2 cents worth. E-mail if you have any questions. Lots of people helped me through my project and I'm glad to pass on what I was shown, for all its worth.