I fyour fuel injected - I would forget the Twin 40's although they do sound great. IF you skimp on anything you will always regret it. If a jobs worth doing it's worth doing right. Gearboxes - Look at Skoda & Porsche for possible donors. Obviously get a strong box from a performance model. Try to match the power output of the donor car with yours when looking for a gearbox so the ratios are not to wildly off the mark.
Little things matter as well. Look at a lighter flywheel, heavy duty clutch and of course racing disks/pads. If it's mainly for street use don't go too mad as lots of performance parts make driving on the roads a real pain.
Agree with Waynne in principle, however;
Be careful when considering using racing parts on a road car.
Racing pads only work efficiently when up to their high working temperature, which will never happen on the road unless you drive like a complete dic*head.
I think that a lot of tosh is spoken about brake upgrades when cars are for road use. If you are keeping to the speed limits
then the standard brakes will be ok (if in good condition) as that is what they were designed for. All a tuned engine will do is get you to the required speed quicker and you don't use your brakes for that
If you are wanting to stop quicker, why? So you can drive closer to the guy in front? Harder pad material means more brake pressure is required so the servo may need attention. You could even end up with longer braking distances when pads are not up to their correct operating temperature. Braking feel can also be lost, especially when pads are cold (and racing ones will almost always be too cold). Talk to a couple of reputable brake specialists (not just shops selling the stuff). A lot of improvement can be achieved by a slight change in brake pad material without going to the extreme of using race parts.
Alternatively, look at what the manufacturer has done to the brakes of his more powerful versions of your car. If they fit your car, this could be a very cost effective way of upgrading if your original system need replacing.
Light flywheel/clutch assembly will allow a faster responding engine due to the reduction in reciprocating mass. However, this can be a disadvantage when accelerating from a standing start (traffic lights, drag strip) as you need the stored energy to reduce the chances of the engine bogging down. Also, due the reduction in the dampening effect of a heavier flywheel, tickover and slow running will not be so pleasant. Again, what you do depends on what you are planning to use the car for and what you are prepared to compromise on.
Heavy duty clutches are what they say, heavy duty and this also relates to the pedal pressure required to use them. Without changing pedal ratios or master cylinder/slave cylinder bore ratios, you could soon end up with a stiff left leg. My Elan (25 years ago) ran 4 paddle racing clutch and the girlfriend (now wife) had to pull on the steering wheel in order be able to apply enough pressure on the clutch to change gear
I have learnt a lot since then