Shaun,
here's my 2 cents ......
By all means cut the tunnel out if necessary but re-use it as well. Section your original tunnel and add strips to increase height and width etc. ie a 2" strip welded into the centre will give the extra width, but will retain the 'moulded' shape of the tunnel. Same for height obviously. In my experience, fabricating a new tunnel is a bit of a job and you will end up with a hard edged structure rather than a smooth flowing shape. If you get stuck I can help you out along the way.
Cheers