[old231]

I'm wondering if the new location of the fuel pump can be too high above the carb. With the gas line filled and the pump pumping it may be too much for the needle valve to handle. It is fighting the weight of the gas, gravity and the pump. Can you relocate it to a point below the carb?

As to the additional return spring we both have, I added mine so the butterfly would close faster and stop the backfiring. The spring on my carb was just not strong enough. It is the original carb and I replaced enough seals. Your new carbs should have a return spring built in. Are you sure you need the additional spring?
And to the linkage length, I'm not sure, but do know that too strong a spring will mess you up. Try it with out the additional spring if you decide drop the fuel pump lower.