Recently purchased a 2010 500c trying to so an Abarth replica, i have a Abarth back box but looking underneath the car and if doesn't look like its a straight fit has anyone managed to fit one?

As long as it will physically fit, then you're going to need to fashion some pipework in order to connect it up to the original center pipe, or, take it with the car to a custom exhaust place and get them to make you a connector section, not sure how much.
Ensure its correctly hung too.

A simpler idea my be this(example) http://www.ebay.co.uk/itm/Exhaust-C...650620?hash=item19f875c6fc:g:LmYAAOSwQJhUha7J

try looking up flexible exhaust pipe on Ebay, note the two ends you have maybe different diameters?

Thanks i wil let you know how it goes