One piece front-end

First if you have just got rid of the metal shroud, metal wings and metal bonnet and replaced them with a nice lightweight one piece front end remember it will be lighter and ride higher so you might want to lower the front back down so that to car sits at least level, preferably slightly nose down.

 For better access.

To start with my front already had locking bonnet pins where the standard hinges used to be.

Preparation
Measure and mark out where the top of the footwell is on the bonnet. Do both sides independantly (mine were nearly 1/2" different from one side from the other).
The lower parts of the wings are then cut off level with the top of the footwell.
I used a hacksaw blade
Rough up the inside faces (bonnet and lower parts) of the fiber glass, from the edge to at least 2" (5cm).

Body Parts (wings/fenders)
The lower parts are then located on 2 pins (could be bolts) at the bottom into the sills, and the top twisted around for best fit and held.with tape (gaffer, tank, whatever).
Then the top is closed off with fiber glass (I put thin plastic over the top of the footwell and after the glass set peeled it off).
When you take the lower part off you can give it a few more layers of glass on the inside.
Now bolt it back in place (2 places). The front bolt is a long one with the thread upwards and then a plastic tube is slid over (to create a bonnet locating pin).

Bonnet
Now a thin peice plastic is put over the new face and around the locating pins.
Then this is fiber glassed, remembering to leave a space around the Wiper Motor (or else you won't open the bonnet very easily).
Then the bonnet is shut and moved about to get the best fit and taped in position.
After the glass is hard the bonnet is released with aid of a thin knife between the bonnet and plastic.

Finishing
Make sure you put a few layers of glass on the inner faces, to be as strong/thick as you want it. Then go around with a file or grinder and clean it up and spray.
 

For better Cooling

You'll notice in the top photo that when I got this bonnet it didn't have a shroud between the bonnet and rad. This means at any speed the engine tends to run hot. As the slot in front of the rad is only half the height of the rad, without a shroud the rest of the air across the width of the bonnet opening will pass either side of the rad, starving it of half it's air. The easy option is to make up some bits of bent ally to duct the air sideways to the rad. You could create a duct that fits on the front of the rad but purely because I think it gets in the way, I fitted mine to the bonnet.
 

	This is a piece of thin ally. A hole is drilled where the bend line intersect. This will stop cracks forming when you bend the sides.
Here it is bent and offered up. When your happy with it you can bolt, rivet and or fiberglass it on. The only important part of this is the flange must line up just inside the rad housing.

 
Obviously you could just as well do this in card and fiber glass it, which might give a nicers shape and might be lighter or make a wooden former/mold and fiber glass that. It's horses for courses, for me it was quicker and easier to do this. All that matters is, is it strong enough and does it work and in this case I can say Yes and Yes.
 

 For better Cooling and Improved Aerodynamics
The Theory

Air comes in through the front then through the radiator, it swirls around the engine and exhaust and looks for a way out. Unlike the song the only way is Down either under the car, which already has it's share of air unless you have a front spoiler. Or through the transmission tunnel which is ok but too much and it will end in the cockpit (blowing through the gear stick gatter, or spilling out under the back. The final result of this lift, drag and a build up of heat.
 

Top Vents

On a tin top (saloon,coupe) the best thing is to cut a slot in the top of bonnet behind the Radiator. Unfortunately in an open top car this could be a bit iffy as any fumes, liquids whatever potentialy end up on you. As I don't run it with the top down this doesn't bother me but it is something to think about.

 

Side Vents

A slot either side above the the footwell in the sides of the bonnet/wings works well.
Some people just put in a series of large holes done with a hole saw and others make it triangular. Put a grill (wire mesh whatever you like) inside to stop unwanted hands going through or you could fail the MOT.

  Here are just a few examples

 

Air dams

If you have a long nose bonnet you may well find at high speed  the car get twitchy/light on the steering. This is usually because it pushes air under the car lifting the front and reducing the pressure on the front wheels. 3 things can help side vents, lowered front springs and finally an air dam. The air dam stops air from going straight under the front and redirects it around the sides. There are a number of things that need thinking about doing this.
If it's too low it will catch on ramps, speed bumps etc. not a worry if this is for a track car.
Check your brakes and exhaust don't get too hot through lask of air flow, I've never had a problem with this but it's always a possiblity.
The one above is very near the bonnet hinge so does not ground when the bonnet is lifted. Others have been further forward and have to be removed to allow the front to open.

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