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?
Yes.
Are all 4150 metering blocks the same?
Pretty much.
Do they sell metering blocks with the lower T-Slot restrictor set up?
Yes, Allstate Carbs

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