Yes, eBox can be run only in web proxy mode. Just active only the web proxy service (you don't need to install the other packages).
However, bear in mind that if you want to use transparent proxy eBox must be configured as gateway.
In your case maybe the better way to configure is to use it as non-transparent proxy and in your firewall machine allow web access only to eBox to be sure that your users pass through it.
Cheers,
Javier