The \$8 price that results in a surplus under perfect competition is just a thought experiment. Strictly speaking, if the market price were \$8, then firms would not only produce 35 million pounds, but also sell 35 million pounds, since this is the definition of a market price. On the other hand, consumers would only demand 15 million pounds. So the surplus is sold, but not to consumers. This of course is a contradiction. It just shows that the market price cannot be \$8. For the same reason it cannot be any other price than \$6.
The model of perfect competition is an inherently static model and is not really fit to answer questions about what would happen if the market were not in equilibrium.
A related dynamic model might e.g. tell you that producers have to produce this period to be able to sell next period, so they have to estimate next period's demand, i.e. next period's market price, in advance. If they (wrongly) estimate this to be \$8 and therefore produce 35 million pounds, then in the next period they will only be able to sell everything if they considerably lower their price. (And here you are right, they would have to go below \$6.) They would then presumably adapt their estimate for the next period and eventually learn the true demand curve. Finally they will produce (and sell) the equilibrium quantity at the equilibrium price, and the static model takes over. But this story needs a more complex formal model and is not part of the standard model of perfect competition.