Idle Transfer Slot on secondaries?

Ok the link with explanation is good and I understand it. This modification moves the IFR to below the fuel line making idle smoother for a lower vacuum system because it is submerged and less affected by air pressure?
I see this modification can be done on both carbs as a typical modification by many people. Saw a video on this as well.

I assume if I mess it up I can get a replacement metering block worst car right? Are all 4150 metering blocks the same? Do they sell metering blocks with the lower T-Slot restrictor set up?

Also what is the reason to use the Holley base plate on the Proform? Is it because of the linkages?