There are several places(JEGS, JC Whitney, etc) that sell different types of sound deadening. material. I used the aluminum shielded fiberglass type under the dog house and the aluminum shielded foam type on the firewall and under the front carpeting. I also used the foam in a can to seal up any air leaks in the firewall which also allowed noise to come in.
Once done, I can hear my wife at all times when we are driving, even if we are in a steep climb. I'm thinking of taking some of the insulation out. :B
Seriously, once everything was sealed, the front end is quiet enough for normal conversation about how much money we are saving with our gasser, compared to the various DP's we used to have.
We do travel out West and get in to the mountains on a regular basis. The amount of time that the engine is noisy is actually very very seldom. Normal driving, like at cruising speed on the interstate, or around town, is not noisy at all. I'm guessing that we eliminated 75% of the noise. I also made sure that I was not getting any wind noise around the window vents, etc.