The UK has fought for the freedoms it has, but I believe that some states in the US have even greater leeway to change things. I've seen US vehicles without mud guards etc, which wouldn't be legal in the UK.
The rest of the EU is much more restricted.
I blame two reasons:
The EU commissioners, unelected politicians who set the direction of EU policy, are very influenced by the German auto industry, who are very against modification of vehicles, or even fitting parts that they don't sell or manufacture.
About five years ago they nearly brought in EU wide legislation to ban modifications, but a campaign to stop this, led by the UK enthusiasts, was successful in killing the idea for a while.
Plus:
The rest of Europe is a lot more compliant with dictats from central authorities, as they are from a culture of Canon, or Napoleonic code law, which can be summed up as follows:
Unless it's allowed, it's illegal.
The UK and the US follow Common law, which is basically:
Unless it's banned, it's okay.
Which is why the Portugese authorities don't allow the freedom to mess with your own stuff.
Sorry, you'll have to dig deep to get your truck into Portugal legally. I imagine you can, but you'll have to get some special route to get it allowed, look into niche stuff, where modifications are allowed, like farm vehicles, fire trucks, mountain rescue, etc.