Adding whole-house surge protection to main panel
First I would like to extend my sympathy for you having a ZINSCO Panel. It is without question the worst panel ever manufactured except for a Federal Panel. However I must say that it has kept many an electrician employed on the replacement of these panels. The breakers have a tendency to weaken and spread over time and that causes arcing and overheating on the bus basically destroying the panel. I would recommend replacement at you earliest convenience.

Now on to your problem. The surge arrestor is designed to be installed on a 20A/2P breaker. If you try and connect it to a larger breaker you are not protecting the Surge arrester circuit small as it may be. In Commercial installations we are required to install in line fuses if we are installing on a larger breaker.

Tapping on to another breaker is allowed but only if that lug is designed to accept two conductors. The only two residential breakers that allow this that I know of are Sq D QO style and Cutler Hammer CH style and definitely not ZINSCO. So you will have to pigtail the conductors of the Surge Arrester to another circuit, then connect it to the breaker.

Mounting the Arrester anywhere on the panel after the Main doesn't have a profound effect on the Arresters use.

Hope this helps.