I'm thinking the exhaust only has one silencer, unlike most cars that have the engine at the front end and space to add 2 silencers the length of the car, so is it the exhaust thats making most of the noise or the engine or just both combined 
Although the Bis has only 1 silencer, being a later car it probably had to meet more stringent noise regulations than the earlier a/c cars. In fact it will gain a lot of quieting from the water cooling - all else equal a w/c engine is quieter as the water absorbs the vibration that is a sound wave.
For additional sound-proofing, you will need as others before me have said, to stop the larger body panels resonating, and you do this by sticking on them sound-absorbing foam.
It's important to realise the distinction between allowing the sound to be created (by the resonating panel) and trying to stop it reaching your ears (for example, by wearing ear plugs or shoving a duvet on the back seat) and stopping the sound being created in the first place (by changing the resonant frequency of the body panel so it doesn't resonate so much, or does so at a different frequency, or preferably both).
Obviously stopping the panel vibrating is the best route, since then not only you but also your passengers can hear each other talk, listen to the radio etc. It also frees up the back seat.
Like most effects of this sort, you'll get the best result if you use the right materials and fit them properly - bonded to the panel across the whole area, not just 'shoved in' or taped in with a few bits of gaffer tape and hope that'll do it.
The right materials are readily available off the roll on eBay or from specialist suppliers - just google 'acoustic foam'. Don't forget that in the engine bay area you ought to use a reflective foil backed foam to stop the engine heat melting the foam.
Also remember that once the foam is stuck properly in place you won't be able to remove it (without a lot of mess and bother) so don't trap any electrical wires, windscreen wash hoses etc underneath it. Also, you won't easily be able to access that side of the panel for any rust repairs, so sort out any rust issues first.
