1970 340 dart

Ok, you need to start guessing. Any idea how long the rebuilt was done? When the motor is cold, best parked outside during the night, do you hear a little piston knock the first few mins when first started / if so it might indeed have forged pistons. If its nice and quite, they are cast or hypers;

Now true, some hot rods are so loud all you hear is the exhaust noise.

Do a compression check also and check the vacuum at idle. You get an idea how the size of the cam and the compression ratio.

And yes, 5,500 should be safe for all rebuilds. Done right, 7,500 is safe. Chances are your motor is somewhere in between.