It seems like 1969 Firebird 400 has different springs (546325) than other Firebirds (3848272), se under "8.013" here: http://thefirstgensite.com/library/partscounter/number/hfh/hfh1.html

I added the extra Volvo springs inside my regular springs, like Bjorn suggested. It took me a while to find the Volvo springs though. Even here in Sweden, Volvos from the eighties are scarce in the junk yards.
Here are a couple of pictures of the springs when they are mounted inside the regular springs.