Thank you guys!

Colin:
Heat still remains in the cabinet.

True.I find rather amazing how many old design types, esp. for consumer
tube gear, were oblivious to heat. I think ppl had a "machinery" or
"engine" paradigm in mind. If stove and charcoal iron are hot, why not
a radio?

WRT boatanchors, I always tried to improve ventilation, with a
suspicion that in so doing I might move frequency-defining parts off
the expected temperature range, but the '390 seems to me quite well
temperature compensated. When Collins folks were designing this, bitter
cold in Korea must have been a fresh memory.

Right now that radio is coverless, perched mid-air on a rack-on-wheels.
I run a room fan aimed at it, and the difference in temperature is very
remarkable. The previous owner, I2FZX (sk) used it for decades with
covers on, sitting almost directly on a surface, with an HRO-500 right
on top, and never had problems with it. Maybe the HRO did.

Longer term, I plan to put it in a desktop wooden cabinet, stacked with
some SSB/synchro-AM device and a speaker (and a thermometer??), with
plenty of forced airflow. I am considering flowing air from the bottom
- out the lower side holes - into the upper side holes - out the top -
sucked out the back by a bank of slowed-down, underfed 120mm fans. I
also want to do the ss-rectifier mod, and introduce a NTC resistor for
inrush current limiting, both of which will cut heat.
_____

Back to the original question, I'll do some experiments with engine
paint and a meat thermometer. If it's good for telling if meat is
cooked, it may help avoid cooking tubes. :-)