Keep pluggin' away...you're fightin' 40 years of "entropy"...
Wow, seven rows clogged...
, no wonder it was running a fever !I'd suggest driving the beast for a bit; get acouple of good half-hour highway trips on it, and see if the black smoke and "loading-up" go away.
As for your temp gauge, double check the connections at the sending unit and the dash unit.
The radiator pressure cap has no impact on how quickly the enginecomes up to operating temperature: that is solely a function of ambient air temperature and the thermostat.
What the pressure cap does is determine at what temperature the coolant boils. At normal atmospheric pressure @ sea level, water boils at 212 deg F. For every one pound increase in pressure, the boiling point is raised about 2.5 degrees, therefore you reach a higher temperature before coolant starts boiling away. (This is the principle on which Grandma's pressure-cooker operates.)
So, with a 15 # cap, this would raise the boiling point of our coolant to about 250 deg. Fahrenheit.
When radiators get old and clogged (and weak) they sometimes begin to leak when the system reaches full pressure; a common band-aid is to replace the 13 -15# cap with a 7 # cap...instead of leaking from the core, the radiator then just pushes coolant out the overflow, if things get hot enough.
As a bit of historical trivia, pressurized cooling systems didn't come into use until about 1950.
I "solved" my clogged radiator problem, and was immediately confronted with a "rusty gas tank = clogged fuel system left me dead on side of road" issue. The car has been up on jackstands for 2 weeks now, with the gas tank out, waiting to get scrubbed-out and treated with POR-15 gas tank system, then re-installed.
yes, it's always something...
Hang in there !