howdy Norm,
Any performance upgrade will be as strong as its weakest link.
Chances are good that with your carb swap alone, power gains won´t be satisfying, because despite the bigger 250 carb (be it a Carter YF or RBS) the carburetor opening on the 200´s intake log will still choke your engines demand of air.
Just my two cents, I may be completely wrong.
BUT:
Maybe there is a more or less commonly available carb adaptor which I don´t know of;
I´d suggest fabbing up your own adaptor from scratch anyway.
I´ve seen people doing this with only some piece of aluminum, hand file and couple yards of sandpaper roll.
That whole surgery is comparable with Slades (cobrasix) offenhauser tripower setup on his 200ci engine; just with the little difference that you don´t make two new outboard holes, but try to enlarge and smooth the original carb opening on the intake log, thus adapting the larger throat diameter of the 250carb.
You could also get a later 250 head for your 250 carb, that should solve most of the airflow and adaption problems. Of course, that swap leads to new problems - compression drops due to the larger chamber volume of the 250 heads...
I´ll try to figure out some adaptor for your 200head-250carb, based upon autolite1100-1V-spacer and carter RBS - spacer measurements. Just for fun, ain´t got nothing else to do tonite...
...later
)
