I don't know anything about your dual carb setup, but all things being equal, I wouldn't worry about it at all.
All I drive and all I've ever driven my entire life are old cars. By "old", I mean pre 1972 cars and pre 1980 trucks. I used to worry like you about their capabilities but no more. Our motoring forefathers and automotive designers weren't as primitive as many people think.
I've driven my vehicles thousands of miles (actually hundreds of thousands of miles combined) all over the country in all kinds of conditions , including mountainous regions with large changes in elevation. I've never had a problem with a vehicle's factory drum brakes, carburetion, or anything else you might worry about.
Your 1959 261 is "modern" enough engine that there shouldn't be any problems at all climbing the mountains.
