I spent some time in your neighboring country Deutschland. I recall the country is very strict about modifications without proof that it was authorized by the manufacturer. The Netherlands less strict?

Leaf springs and a straight axles are extremely heavy. Shipping to the Netherlands would be equivalent to buying a luxury Mercedes (many Euros). If you have friends who are familiar with vehicles built for rough terrain (Jeep, Land Rover etc.), ask them about shops that do straight axle and leaf spring repair. Also you may want to contact this company in your country http://www.parabolicsuspension.com.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."