This is why I don't like those carbs. I've been on the dyno with them and they won't open the secondaries until way past 7k. That was a big block Chevy. Most of the time they won't open until it's way too late, even with the lightest spring in them.
I've got a Holley 650 double pumper with mechanical secondaries from a previous 340 many years ago. Worked great in the 1973 Duster it was in. A rebuild kit is about 80 bucks. Option number 2.