Thats a half lie, it will fit cause we've done it with a Peco backbox onto my mates Mk1 Ph1 1.2
Only problem being you have to fiddle with the mounts a bit and it is wrong size where it joins, so blows a fair bit. In his case, it didnt matter too much cause it doesn't make a noise when blowing and actually makes for a nice raspy note around 4k.
Just make sure it's well tight around the join, and use fair bit of exhaust putty, which we used none of lol