The Racer Brown cam you have looks like it has 238 degrees intake duration at .050". The 260 H is 212. A big reduction.

238 degrees isn't a big cam for 500". I would expect it to pull well from 1800-2000 RPM and hit its sweet range from 3000-5000 RPM. It may be a little sketchy accelerating below 1800 RPM with a manual transmission. I wonder if there are some other issue affecting driveability as others have posted. What are the drivability issues? Was the carb swap done to improve the situation?

If you can post some more details it may help.