For twin DCOE or DHLA applications fuel pressure should be around 2.5 PSI. Any more will create pressure in the float bowls and result in poor low rpm running i.e. bogging and flooding.
As Giles says, you should be looking at a Facet red top pump combined with a filter king or similar, which will reduce the pressure accordingly and keep the fuel clean.
Trumpet length can have quite an effect on performance as the airflow greatly depends on them. Go for as long as you've got room for, leaving an adequate gap, usually 25mm between the trumpets and filter. At the end of the day, space is usually the main influence on trumpet size anyway
The dellorto DHLA does suit vauxhall engines better, as the accelerator pump mechanism provides an atomised mist of petrol rather than the plain squirt of fuel as with the Weber DCOE