Well Chas, I am no cam expert, but I know now that the the vacuum produced by the engine has a lot to do with the jetting and venturi size.
In my case, even with my engine having worn
oil rings (the rest is ok, with 175 readings on all cylinders in a compression test) the vacuum produced was fairly high.
If you have a strong vacuum at low rpm you can go for a larger venturi size and still have enough signal to produce the correct fuel/air mixture.
When a cam has overlap it means that the two valves will be open at the same time during the final exhaust stage, which helps scavenge the exhaust gases while drawing from the intake while the piston is still moving up. That helps fill the cilinder better, but this works only from certain rpm's on up. At low rpm the response is pretty crappy, and that is why the idle with those cams is so bad.
The vacuum with those cams is comparably weak at low and medium rpm and the velocity of the air in a too large venturi will not be enough to draw the correct amount of fuel from the spray nozzle, creating bogs and hesitations in the transition.
At high rpm the engine will suck in enough air to make this work, that is why they say that for racing the larger venturi of 42mm is ok but your low and medium range will suffer.
I do not know how your cam will behave. It is not so far off my cam.You can try taking the venturi out and turning the inside to produce a 38mm innner diameter. You can try and see what happens. If it still works ok, you bore the venturi out to the next size and see again. The worst thing that can happen is that you have to buy new chokes if you overshoot.
IMHO I believe that anybody with 48IDA's on a 427 with a mild cam must be running rich with the more popular jetting and choke (37) sizes suggested in several threads in this forum.