Regarding the rules for "motorredskap" the easy and correct answer here is that it you truck will sadly not longer fullfill the requirements to keep the classification as this, and thus have to be reclassed into a "campingbil" or a "personbil over 3500kg". The rules and requirements have changed a bit over the years, but the way this is and have been practiced the last years is that no new truckbased vehicles are being registered as "motorredskap", but as regualar trucks (N2 og N3) with various special mounted equipment. This also apply to older vehicles that are being modified or converted from their original apperiance. I know this as a fact, as I am working with truck mounted special equipment for a living as a mechanical engineer for a Bodybuilder Workshop here in Norway.. (
www.amv-as.no). When you convert the truck the weight and body that is mounted wil be diffrent from the original firetruck body, and the main purpose for the truck will thus no longer be fire fighting... Ergo it will get a different clasification that matches the body and intended use. For instance my Unimog went from a "Motorredskap" to "terrenggående personbil over 3500kg med campingpåbygg" when I went for the reclasification.
....But, I also know a lot of people who just modify and drive with out any modification in the registrations papers. The downside then is that if you get stopped (and at some point you will..), you will be in a lot of trouble and have a big hazzle to sort things out in a rush..
If you want you can contact med on PM and I'll be happy to give you a call and explain a bit more details around all this
Regarding the wheelbase and total lenght of your truck, my Unimog has a WB of 3250mm ant total overall length of 7,26m from the tip of the winch to the rear of my spare wheel, this does not feel very massive to manouver and we have managed to get around small Italian villages without any drama