If it is leaking anti freeze then your heater core is bad and you will need to remove it and either get a new one or take it to a radiator shop for repair. It's not likely that you will ever get a heater that will clear the side windows. While you have the heater out of the truck clean those windows real good they probably have an anti-freeze film on them and then get some rainx and give them a good wipe-down. That should help with side visibility in bad weather.
The heater is a pretty simple piece and you should have no trouble taking it apart and seeing how it works.

1953 Chevrolet 3600
Fried Green T'mater (Vern)
In the Stovebolt Gallery
Caution: Political Statement
Man who runs in front of car gets tired, man who runs behind car gets exhausted.