As edski reflected, that Olds 2bbl will have two kinda big primary venturis. The primary of your Holley is only a 1-3/32 venturi, about 220cfm or so from the primaries only - smaller than that Olds 2bbl. So if the 4bbl is too much for the stock 250 (I have the same situation) maybe just block the secondary SHUT on the Holley and you have a "free" 2bbl with no adapters, no hassle, and you're ready when you need the extra airflow.
All the jetting I can find on the 450-465cfm Holley ranges from #50 to #53 in the primaries so you might see what you have and base it on that.
About the only reason to go to a 2bbl would be if it was a progressive Pinto/Weber style 5200 carb. They flow about 203cfm total (4bbl style rating at 1.5" Hg)and are a progressive 2bbl so you cruise on the tiny primary 1bbl. Great carb for response and mileage. A tad small for even your stock 250 but then again the 470 Holley is quite a bit too big.
