I'ma guessing it's kind of what I expected?
I ~thought~ I saw some brass-ish sparklies in the drain pan, but perhaps it's residue left over from rebuilding the blown TH350 this summer.... Not really any brass in a SBC.
High-ish iron content might be the flat tappet cam getting flatter, with 115# (closed) 340# (open) valve springs.
Coolant may very well be the steel shim head gaskets heading south again. I know it got a hot a couple of times when the electric fan switch stuck open....
Thoughts??
Keep up with the miles and send it off again in 2000mi and see if the levels are still high. 2 data points is the minimum to draw a trend line. Maybe put in a magnetic drain plug, or around the oil filter?
Is that SBC iron block and did you drain the oil while you did the ATM rebuild? Couple times I've gotten high iron readings after I left a motor sitting without oil for a while as part of a refresh or swap project. Goes away in a couple oil changes, with improvement shown at the second change. Coolant number wouldn't code brown me but I would lather, rinse, repeat for sure and start thinking about making time for an HG replacement if the next one came up bad.
Yeah, I'd want at least one, if not two more data points to really worry about iron content. I'm about to turn over 12k miles on a recently rebuilt SBC with similar iron issues, but I also have very high lead values. 
The big downside to knowing all this stuff is it gives you that much more to worry about. Otherwise, I'd never have given it a second thought.
Hahahaha, true, that.
It's a 3970010 block, flat tappet cam. I drained the trans into the same pan, but I thought I had cleaned it out before I drained the truck oil. The sample was taken mid-stream, not from the pan. I did not drain the engine oil during the tranny rebuild, but it ~was~ down for about a week. The truck is my daily driver.
I was curious about the cam, but I think it's still surviving. I use Lucas' "Hot Rod" high zinc oil.
I likely won't change the head gaskets until I notice a more significant problem. Last time I blew the head gaskets (two years ago?), it never changed the oil visibly at all.
Those are all interesting observations! For the last ten years or so of my employment before retirement, I operated a reduce risk management program for a major US automotive producer. We used oil analysis, infrared, and audio scans, to reduce our failure risks. What we discovered, which tied into my decades of oil analysis tests in the railroad industry is that, long term we can initiate many processes that are of huge monetary value. But short term, as in less than forty years. you're not going to find anything of value!
What I'm saying is, enjoy and use your ride! THE WORST THING YOU CAN DO FOR ANY RIDE IS TO LEAVE IT PARKED!! Bottom line, just change your oil on a regular basis, and when you start your car don't let it idle in your garage but drive it long enough to get every part including transmissions and rear ends of it truly warmed up, and drive your car often enough to prevent old fuel issues!
Neither I nor you likely won't own or operate a car long enough to have long term issues unless we leave them parked!!
I totally agree for my daily drivers. But I have some cars that I run near redline A LOT at the track, so I'm vigilant for iron, lead, copper, and overcooked oil.
Update (only because another Blackstone labs post reminded me of this):
The coolant evidence in the oil may very well have been leaking intake manifold gasket.
It had been "eating" coolant for some time, but it started getting worse beginning in the summer, and finally declared itself with a puddle on the garage floor at the back of the motor. Thanksgiving weekend I pulled the intake off, and was much more meticulous in sealing it that apparently I did during a dead-of-winter-all-nighter-head-gasket-swap-to-get-to-work-the-next-morning repair.
Back of the passenger side head was wet, there appeared to be some wet in the #8 intake port.
Currently - no coolant being lost at all.
This leaves the high iron content likely just the result of the snotty flat-tappet cam and high spring pressures. I'm ok with that. The cam is "Potato Therapy" to me.
The sodium in that is from the oil. 300-400ppm in gas engine oils usually is part of the add pack and IIRC, several lucas oils use that. High iron in an all iron SBC is to be expected. Now, if you were seeing potassium along with the sodium, moly and boron, then yeah, it'd be the coolant. I definitely would not have attributed the Sodium to coolant in this case.
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