Get a compression gage, and take readings from all cylinders with all the plugs out, the engine warmed up, and the carb wired open at full throttle. Post back your results.
"rebuilt" doesnt mean "done right". And back in the days where "hemi grind" cams were used a lot, just buying the right parts didnt guarantee the thing ran well. I pulled apart a "top of the line" 440. It had a dual plane aluminum intake, Ultradine cam, TRW domed forged pistons, and "all new valves" with maching springs. The pistons were way down in the bores (so compression was really 9.2:1), the honing was wrong for the rings used, so they never sealed well, the pistons were loose in thier bores by .003", the cam was isntalled by lining up dots, so when I checked, it was retarded 3°, the seats werent installed properly in the heads, and the valves were reused, along with springs on seats that were not cut to fit big springs. All decent parts, and all poor execution.