thinking about cutting up the cap and filler tube

right now I'm waiting on a new filler tube grommet in the mail and I was at the gas station last night, filling up my truck. I think I saw someone on here that modified their gas cap area to put a bigger or a new gas cap on their car. well the question is, how well do you think a modern style gas cap box would do in my 65 Barracuda? In the trunk it looks like there's enough room in there to maybe put a box in there and I think I could cut my filler tube a little bit for it. any thoughts how good of an idea this is?