Hi Astro... you have a couple of choices... You can go with any or all of these. I design custom car interiors and this is what I've done in the past... I've just finished my own 120Y and the difference was tenfold.

1: put a sound deadening mat on the inside of the bonnet and on the firewall. I have on my XR2 Turbo Convertible and that made it as quiet as a mouse.
2:install underlay prior to putting in the carpet. Use the hessian stuff, not the foam (fire resistant)
3: use Dynamat and put it wherever you can afford to. I say this because it is very expensive (not worth the money IMHO)
4: use cloth materials wherever possible, It absorbs road noise (which is about a 200 hertz frequecy) well. eg - cover the cardboard kick panels.

If you follow these steps, you should make a difference. I hope this helps you out.