All live axle Commodore tanks fit. IRS up to VS fit but I think you need to modify one of the straps. I have a VN V8 tank in my VK, bolted straight in. Need to cut about 40mm off the bottom of the filler neck to fit. My fuel guage is working correctly as well.
Yeah if your going to do your fuel tank try and source a 80Ltr drop tank. Fuel tanks aren't much fun to play with so you don't really want to do it too often. I recently installed a 80Ltr drop tank from a VQ statesman into my VK and I don't want to do that again in a hurry LOL. Only other thing is if you put in a 80Ltr drop tank you will need to install a return line.