Redline sell them as Holley 4-bbl spacers. You just weld them up to the 2-bbl intake like
Tassie 2V or
Mustangaroo have on there 2V intakes.This is a very good way of making a cost effective 4-bbl conversion.
Dead easy to hack up a lovely Ultraflow intake.
Here is a 4-bbl to 2-bbl adaptor.
You just make it the other way around, out of tooling plate or free machining alloy, and have it TIG welded to the ultraflow.
Since we are not using methonal, and not at the oval track, a 2-bbl utraflow is too small. There used to be a special Holley 650 2-bbl, but they haven't been made for years.
Today, a person wanting 300 hp from a six needs to
a) create a 2-bbl 650 cfm carb by getting Barry Grant 62.5 thou over throttles, fly cutting the venturis out another 62.5 thou, and then eliminating the choke and fitting the rare #6425 air horn. They were able to run well over 300 hp on petrol
b) Spend big money on the new breed of specialist carbs from the States. Today, these can be used. This one below has the venturi taken out from 1.375 to 1.59", and has the throttles taken from 1.6875 to 1.75"
dAM 650 2bbl
"The Switch Is On..."