Interesting that you should say that Jimbo - I'd assumed that my carb set-up was original, but this is my first Vette and with a car nearly 50 years old you can never be quite sure what might have happened to it during its long life. Certainly the carb is the correct one for the motor and year and the pull off looks as if it's never been off, but apparently the correct pull off for this carb looks like this....
View attachment 3005
...which is nothing like mine. I'm wondering if your original looked the same? (assuming we have the same motor - 350 base/270 HP?)
Any way the plot has thickened somewhat since the reason I originally suspected a possible dodgy pull off was a severe tendency for the engine to 'bog' when quick acceleration was demanded. This I attributed to possibly the acceleration pump or the pull off not working and checking the pull off with a vacuum pump produced no movement. Aha, I think, that's the problem! However when I reconnected the vacuum line yesterday and started the engine to check the acceleration pump, lo and behold - the pull off now works!
So, for the moment I've ditched the search for a new pull off; I've ordered a rebuild kit for carb and when it arrives I'll strip it down, clean it up, replace the acceleration pump and see if the overhaul improves matters. Hopefully I won't have to follow your example and invest in an Edelbrock, but it's good to know that works for you.