Hi,
I have the setup you are asking about. My 3a has the small port 4a head in it so I wanted the matching 4a intake. I use the H6 carbs that came with the car. Heres what I found:
1. Stock air filters will hit the fender well. I made a set of filters with tapered K&N filters. I offset them vertically about 1". I have about 1/4" fender clearance. Adjusting the fwd carb jet nut is a pain.
2. The stock throttle linkages binds up. My manifold is about .62" longer than the 3a manifold. I also think the mounting angle is different. I went to a cable.
The HS6 carbs would fit a lot easier. The H6 carb is 5.5" long, I think the HS6 is somewhere in the 4" range. I installed a header at the same time I swapped intakes so I did not get a direct comparison. It does run well.
Good Luck, Roy