I know that the prefered venturi size for a MC2100 is 1.08. Would a 1.14 size work for a 258? I can't really tell for sure I have seen charts showing cfm for the different venuri size for theese but every chart is different. Very different. Anyone running a 1.14?
I had someone say they did use a 1.14 and it worked fine. I'd be hesitant as oversized carbs (and the 1.08 is already oversized from stock)can cause problems such as loss of low end response and poorer fuel economy from having to squirt so much from the accellerator pump to make up for it. Make sure the carb has the pcv fitting and ported vacuum, otherwise don't waste your time on it.
Best source for a 1.08 is an amc 304. Hard to find though, and I have none to spare.
Another source for a 1.08 is a ford pick-up with a 302. If you need a 1.08 I have a fairly brand new one, I've never used it, owned it for 1 1/2 years, and the rebuild tag says 7/00. Make me an offer if you are interested.