A bit more information. The top nebula NGC 3578 is about 6000 light years away. The lower NGC 3603 is 20,000 light years away and we can only see the bright central part with its very young (one million years) bright star cluster as the rest is obscured by intervening dust and gas. It has been estimated to be about 1000 to 2000 light years across and would be the largest HII region in our Galaxy.
More info here
http://www.atlasoftheuniverse.com/nebulae/ngc3576.html
So I would put the (super?) nova at somewhere between these distances. Most probably nearer 20,000 light years.
Bert