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. 

+1, As noted above, NO.   6AJ8 tubes cannot be safely substituted for 6KA8 tubes. The two tubes have different pinouts and internal structures: the 6AJ8 is a triode-heptode, while the 6KA8 is a high-mu triode and sharp-cutoff pentode.

Using a 6AJ8 in place of a 6KA8 could result in improper operation or even damage to your equipment. Please be careful.  

Thanks to everyone for clearing this up for me. I really appreciate it.

@oddiofyl +1

 Lord! I just got a flashback of spending hours with my dad on a Hickock tv10 testing tubes. Ahhh, those were the days.😁

A lot of good equipment has been hurt by the wrong tube.  I always use exact spec when possible.    

Brent Jessee' website is a valuable resource also.   Lots of info there.  When i think about it that's where I'd start .  I go there occasionally,  I have gear with obsolete tubes