Most disc brake E bodies used a 1" diameter bore master cylinder in 1970. The exception was hemi cars with disc brakes which used a 1-1/8" bore master cylinder.
All '70 drum brake E bodies used a 1" bore master cylinder, however, this master cylinder was completely different than the disc brake part.
The '70 E body disc master cylinder was a 1 year only part and very rare to find today. There is no equivalent available on the aftermarket, nor is there a kit available to rebuild an original. However, there are kits available with correct seals that can be used to rebuild an original MC if the original's pistons and springs are in good shape.
In the past I've used a 1-1/8" master cylinder on my non-hemi '70 disc brake Challenger and it worked fairly well- as mentioned, the pedal was a little hard and I found the rears were quick to lock up if I braked hard. I've also used a '71-up disc brake master cylinder which has a 1-1/32" diameter bore and it worked pretty good. I've currently got a rebuilt original on my convertible and no question it's the best overall of the 3 on my car but the 1-1/32 style is the best 2nd choice if you can't find an original 1" bore MC.
Whatever you do, don't put a 1" bore drum master on your car, which is what I've seen a lot of people do. There is a much lesser fluid capacity in the main reservoir and you risk not having enough fluid to operate your calipers as the pads wear if you aren't diligent about keeping the fluid level topped off!
