If I may say, I wouldn't start welding until I knew what I was welding to. In saying that, with the amount of rust you have, I would suggest sand blasting the floor, rockers and your trunk "gutters" (as from what I can see in your pictures). That way you may find more areas that require repair now, not after it's been painted. Ah.... Do you have a sentimental attachment to the car? To be totally honest, the car looks fairly rough, and just wondering if there isn't a body available at a respectable price that is perhaps more sound.
I'm doing a complete custom, frame off, job now and the brake system is all after market. The booster and master system will bolt to the firewall. That being said there are many kits available that would allow you to mount the system under the floor. It just depends on what you want to achieve.
If your interested I could get you a list of suppliers that would be able to get you the product that you are looking for.