Super tuning car runs good shooting for great

12% is an awful lot.
If it was mine I would be doing something about that!
Here is a quote from Smokey Yunich;
"If the average is over 8%, you don't have an engine", and elsewhere,I read;
"only 1/3 of the heat energy(pressure)developed by each cylinder is actually going to push the piston down.So,if 8% is leaking away, you are theoretically losing 24% of the recoverable flywheel horsepower."

Ok so what is 24% of 8 cylinders? Yup, about two cylinders worth. On a stock 360 say @ .7 hp per cid,252hp, this would equate to 60 horsepower.And this 360 is now pulling like a teener. And not even a good one at that......192hp.

In your case, if that pressure is going into the crankcase, it will blow stuff out wherever it can. Fix one leak today and it will just pop out somewhere else. The oil pan end rails are the second easiest to push out. While the dipstick is usually the easiest.But that little hole won't pass much air.After that, I guess the dizzy is next.And then the intake end-seals.
Make sure your VC breather breathes in both directions!
While the cold test may not be accurate, in your case, at 12%, it won't make much difference.
But perhaps it is just carbon on the valves. This can be usually be knocked off without any more teardown than popping the Vcs off.I just spank the valves while performing the test and watching the gauge.Just a little tap,tap,tap, and let the springs slam the valves into the seats.If the leakage does not change, or changes very little, then, if it were mine;apart it comes! I would be looking for numbers under 4% and under 2% is best. With plasma moly file-fit rings and new heads, I have seen numbers so small that I couldn't really assign a value to it on those little tiny gauges.
Don't forget;the LD test is done with the pistons at TDC, so you cannot open the valves very far, else they will be banging into the crowns, so just a little spanky-spanky, and let the airpressure/springpressure do the job.