ALOT!How will this effect road noise?
ALOT!
Think of a muffled tin can. Not something you will enjoy over an extended road trip.
Went back and edited my post DynaMat makes a product called DynaDeck which will quiet the noise and is easier to clean than carpet with embossed pattern. It comes in a 54" wide roll that you can cut to fit. It can be removed easily if you want to. Vacuuming is much easier than carpet.
http://www.dynamat.com/automotive-and-transportation/automotive-restoration/dynadeck/
And because I am a big believer in Monstaliner and their Chassis Saver product.
http://www.monstaliner.com/
http://www.monstaliner.com/monstaliner_how_to_stop_rust.htm I like the silver chassis saver as it contains aluminum. Then top coat with Monstaliner.
If you use any kind of degreasers like awesome cleaner, simple green, or purple magic types then you have to do extensive rinsing and wiping as it effect adhesion. Brake cleaner works best and leaves not residue. I do not do a lot of surface prep as rust helps adhesion. Just get the dirt and loose rust knocked off.