The usual conversion for a classic ganache is made with equal parts of chocolate and cream. I usually follow this basic recipe and use 8 ounces of chocolate to every cup of heavy cream. If you would like to get a stiffer ganache, just add in more chocolate as you go. I've made eclairs where I've added about 12 ounces of chocolate to a cup of heavy cream so they would set and could be stacked. Since you are whipping it, though, I would add maybe just another 2 oz of chocolate and use a hand held or stand mixer to whip it until it's light and fluffy.
The ganache should be fine whipped at any time, just remember that whipping it will make it fluffy and paler, versus the ganache that is more like a chocolate sauce and sets in a firm glaze.
Hope this helps :)
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