hi hobbo, the commodore tank does fit easily though there is a fair bit of messing around depending on where you want the filler neck to be. the neck on the commy tank comes out on the drivers side so you will be looking at some body work if you go this way.
i got the neck removed and shifted to use the original FC position then used bits of FC and commodore necks to get the end result. fair bit of mucking around as i had to get the tank welded by a professional without the car being there.
the only reason i went commodore was because i had a deal going with a local holden wrecker and it was pretty much all he had which i thougt i could adapt. have a look around some of the jap wreckers if you can you might find something else to fit easier.
the other thing to remember is that the FC fuel guage wont work with this modification as it operates at diferrent ohms.
cheers
dean