Is it possible to change the way Terminal displays files' permission? As you can see, they are -rw-r--r-- and so on. I would like to change it to 644 or 755. How could I do to change it? (Please, this is not a question about sudo chown.)
To change the display of file permission to the octal system, you need to modify the ls command. There is a nice tutorial here: http://agileadam.com/2011/02/755-style-permissions-with-ls/

in short: edit the profile for your shell, f.e. 

nano ~/.profile


and enter 

alias lso="ls -alG | awk '{k=0;for(i=0;i&lt;=8;i++)k+=((substr(\$1,i+2,1)~/[rwx]/)*2^(8-i));if(k)printf(\" %0o \",k);print}'"


reload the config file with

source ~/.profile


and you have the new display type with the command lso