It is kind of weak even for a 307. My 400 sb tested about the same and I found it to be in need of an overhaul when i disassembled it. Here is one more thing to check. And it will be easier if the plugs are still out. Pull off the distributer cap.Place a socket on the crankshaft bolt and turn the crank until the rotor turns. Mark the damper at the end of the timing tab in the direction you were turning. Now turn the crank the opposite direction until the rotor just starts to move again. If your mark moved more than a 1/4" you have enough play in the timing chain to lose power and cranking compression.
As far as being rebuilt, it is a phrase that everyone interprets differently. For me it is a fresh engine, all castings machined, all moving parts replaced with new. To some it means rings, rod bearings and a lap of the valves. Unless you see some proof you have no idea what has been done.