My car currently has the front clip off and I am detailing the under hood area. The heater core is still in the car and was working fine. I don't know if it is the original or not. I do know it was not leaking. After reading several posts on heater core quality, would you replace the existing core while I have things tore down or use what is in the car as currently there is not problem with the core????? Thanks for any opinions.

The core comes out from the inside so it shouldn't matter either way. I would replace the fan motor for sure as that's the problem child with the fender on.

If the heater core was leaking, I'd change it for an AC/Delco core, or at least one Made in USA.

It's recently been pointed out here that while it's true the heater core comes out from the inside, the nuts you have to take off to get the inner heater duct off are on the firewall and can be very hard to reach with the fender and inner fender on.

I would take the heater box off the firewall and at least inspect the core closely for any sign of drips or bad soldering. Mine was no trouble prior to rebuilding the car either, but duing reassembly, one of the hose nipples cracked and started leaking. Thank good ness I fired the motor while the clip was still off and saw the problem!

If you see any concerns with the core, you might be best off to simply take it to a rad shop and have them redo it.

It's recently been pointed out here that while it's true the heater core comes out from the inside, the nuts you have to take off to get the inner heater duct off are on the firewall and can be very hard to reach with the fender and inner fender on.

I would take the heater box off the firewall and at least inspect the core closely for any sign of drips or bad soldering. Mine was no trouble prior to rebuilding the car either, but duing reassembly, one of the hose nipples cracked and started leaking. Thank good ness I fired the motor while the clip was still off and saw the problem!

If you see any concerns with the core, you might be best off to simply take it to a rad shop and have them redo it.
What he said. Inspect it while it is easy to get at, or you will regret it later.