at the end of your black intake thingy (i assume thats what it is) maybe you could put a funnel type thing on to try and get a little bit more air in?

twin carbs sounds a lot of work to me mate, difficult to get a manifold and possibly hard to fabricate and get them to run.

maybe you could polish the inside of the manifold so the gases flow better

and the head if your feeling adventurous, or if you cant drive the car (got the car before you passed your test!?)

then some form of heat wrap on the manifold and other hot areas (the rest of the exhaust for example.

then id look at lowering kits (or chop your springs) not dangerous before anyone says, my dads done it on his motorbike and handles better now

good tyres and good brakes also

i realy love projects like this, cant wait to get my car