Can 6AJ8's be safely substituted for 6KA8's?

NO. Not the same tube.  Pinouts are not the same.

6AJ8 TRIODE - HEPTODE

6KA8 HIGH - MU TRIODE - SHARP CUTOFF PENTODE

Agree with @jea48.  They are a little similar combining a triode and multi-grid unit with a shared cathode, but they are not pin-compatible.

https://tdsl.duncanamps.com/show.php?des=6KA8 doesn’t show a substitute for the 6KA8.

I’m not familiar with either tube.

The best resource for tube substitution in my opinion is an owner's manual for the later built Hickok testers.    It helps confirm pin out, heater voltage and current.  Some have tube substitution charts for the more common tubes also.