I did this to my vanagon, and was mostly disappointed. The single most
importiant sound deadening proceedure that you can do is to buy quiet
tires. Without  these, any efforts at insulating the walls and floors will
fall short of your most limited expectations.I mean tires pumped up to
48psi are hard rubber sound transmiters.15 inch tires at 35 psi are the
standard that most people had in previous vehicles and the standard which
the measure and success.

I put in 200sq ft of r-blox sound deadening,50 sq ft of under carpet sound
deadening from a Dodge caravan.1/2 inch thick neopreen foam rubber 4ftX9 ft
on the pop top,same on the radiater fan schroud,all the inner door pannels
and side pannels with 1/2 inch foam rubber under fabric,spray sound
deadening in front doors and else where,a/c cabinet sund proofed w/b-quiet
extreem, blah blah blah. Little effect on sound deadening did all this
stuff have.

I would spend my time and money on qqquuuuiiieeetttt tires,and 15 inch
wheels so you dont have to inflate them over 35 or so psi. Search Larry
chases tire chart for the quietest tires avaliable and then if you have any
money leftover , buy whatever you want for sound proofing. You will like
whatever you then buy..