What are you going to use for an intake manifold? The HS4's won't fit on the HS2 manifold, and the "B" manifold doesn't fit the 1275 head. However, you <u>can</u> cut a section out of the B manifold, and weld it back together to fit the 1275 head.
You will also have to install new needles in the carbs, as the "B" needles will be way to rich for a 1275.
Are you going to modify the head, change the cam, install a header, etc. to take advantage of the increased flow of the HS4's?
As you can see, it's not just a bolt on swap of parts, and I'm not convinced it is worth doing on a car that is only to be street driven.
I'm not discouraging you from doing it, but I think that at best, you will see only a very minor difference, if any at all, without further engine modifications.
Jeff