Ok....the breather hoses that join at the back of the airfilter & go to the horn's can closed off...it has worked on some cars not all! if you find that closing the pipe causes problem's you could drill a hole in the Bolton filter plate & fit a (don't know the word) "thing" & have the breather pipe going to that. I beleive that you only have to do it for one set of carbs.
I've had a look at the A/C pump on my 1.7 16v engine & another solution would be to cut out the obsructing bit of the air box.
This would mean the airbox can stay & you keep the engine looking orignal (like that matters when you adding A/C)
OR you could try heating the airbox plastic & moulding it to the shape of the back of the airpump. I don't think that would work but could be worth a try if your thinking of buy bolt-on filter's....BTW They sound fantastic
2x 145, 4x 75, 4x Sprints,a 156, 146 now a 155