I had a Jeep Grand Cherokee converted to gas with a certificate and my insurance company never asked for it so some do some don't.
I am going to convert my P38 4.0 to LPG myself over the next few weeks, I might welll get it checked out after so I can get the certificate but that would only be for peace of mind, as I will be following to code of practice when doing it i.e.the location of the pipes and the spacing of the clips e.t.c. I did notice when I had my jeep done that not all of the guide lines had been followed to the letter.
I do agree about the comments about single point it will drift out of tune and when it does say good bye to your airbox as the back fires can blow them to bits, I had to replace 2 on my Jeep, I then used a cone type filter which allowed the explosion to escape rather than be contained in the airbox.