360 hesitation and

comments in the quote

I just broke in the cam on my newly rebuilt 360 and it
starts right up,
idles beautifully and
will run a high RPM no problem.
These comments indicate that;
the gas is probably OK,and
the low-speed circuit is fine,and
that the power system is also functioning reasonably well.
And that the rotor phasing is OK,
and that the Ignition system MAY be ok,as well.
I was told to set the timing at 10*. Today was the first time I attempted to actually drive it which was a test drive around my development, didn't go far before returning.
Its sputtering and backfiring from the carb under load.
This tells us that;
the gas may be contaminated or oxygenated or just stale,or
the power system is faulty,
or the plugs are overheating,
or the ignition is crapping out.
The pic of the one plug seems to indicate that the fuel is OK, that the heat-range is close,SO, to rule the plugs out; What heatrange are they?, or what's the plug number.
The car had the same carburetor and distributor prior to the rebuild.
Details.
1971 Plymouth Scamp
360 engine with close to factory spec cam
1406 Edelbrock carburetor
Proform Distributor

Click to expand...

Those plugs do not look fine. But they appear almost new. Are you sure the gas is pure gas, the same fresh stuff you ran with the old engine? Is it oxygenated?
Since there is no black on them whatsoever, I suspect the secondaries are just lean.
So now we just need to determine; is it fuel or is it ignition.
To do that, you can kill two birds with one stone, by disconnecting and blocking the secondaries closed, then see what the primaries do.
If it does the same thing, back out of the throttle just a little until it stops. If the car surges ahead ,then it is very definitely lean, she wants more gas on the primaries, and you need to fix it. You may have to try it several times to learn this trick. If it surges ahead, then it wants more gas. How far you had to back out gives a hint as to how much more gas it wants. Try this at various rpms starting no lower than 3500rpm, and no more than 4500 for now.