I ran a 2 barrel rodchester on a 261 years ago with modified stock manifold with good luck. I used a 2 barrel from a 283 and had to rejet the main jets smaller. At the time I was doing this smaller jets couldn't be had so filled the old jets and redrilled them to the proper size. Also the old 261 had the head shaved for more compression and had a mild cam that might have helped with the extra fuel air mixture. Sounds tome like you may have had leaks or to big of a carb.
