The issue when making the rear bumper closer to the car is that the side sections (following the line of the rear quarter panels as the car widens) will move away from the centre part as they are pushed forwards.

Demonstrated below you can see the main center section of the bumper remains fixed in its new position and as I push the sides forward to align them, they move away from it.

The one thing that has made the S3 the least desirable of the Alfa Spiders has always been the enormous bumpers added to meet regulations for the USA market as some people think they are not kind to the aesthetic of the originasl design.

In the US, they are hiding small hydraulic rams that will absorb low speed impacts (5mph) without damage, but elsewhere in the world these are excluded and there are just solid steel brackets for the bumpers instead....but they still kept the big bumpers.

Personally I would like the rear bumper to sit in a little (not all the way) just so it doesnt look so ridiculous. I think the front bumper looks fine as it is.

On this car I want to bring the bumpers in around 40mm. This will close the gap between the bumper and tail of the car and make it look more like the front.

First I'm going to remove the edge that is under the rubber trim on the bumper sides. This will allow them to sit close to the car and close the gap between the sides and rear section. An unintended benefit is that the rubber rubbing strip becomes less of a gap filler and it gives a sleeker look. imho

On the trial fit below you can see that by removing this gap the sides and rear realign in the new position closer to the body.

This is the section I am removing. I've seen attempts to cut this at an angle and then recreate the shape where it meets the wheel arch, but I judge this is the best place to cut while maintaining the correct shape. It does of course need to be cut with some accuracy. Here I'm using a hand held multitool.

The result.....

Normally to join this I would use plastic welding staples, but as you can see elsewhere on the site, you cannot easily use that method as the bumpers are thermoset plastic.

On this car, I tapered the two meeting faces on the front for filling, then I shaped a thin alloy sheet and bonded it behind the join with JB Plastic Weld (after thoroughly preparing it) to support it.

The pics below show the bumpers refitted and repainted in silver and black as they should be.

Alfa Romeo Spider S3 Bumper Shortening / Cutting