Is there an aftermarket LED or Halogen low-beam T3 sealed headlamp that fits a 1968 Olds 98 available? If so, would the existing cable harness be adequate to handle the load?

 

PN's H5001 is for a high beams and H5006 are for low beams if you just want sealed beam replacements. You can use the stock wiring for them.
If you want reproduction T3's then try these, they are available also through assorted repro parts suppliers.
Reproduction T-3 Headlight Bulbs (Sealed Beam ), Small Bulb Kits, & Fuse/Flasher Sets for Classic & Muscle Cars

 

Here's a great lighting vendor whom you can check out for possible options

https://www.danielsternlighting.com/.../products.html

 

If you want a correct T3 lens with improved illumination, the only thing I've seen that works is gluing the T3 lenses onto a H4 halogen reflector, as written up in Rod and Custom in 2010.

https://www.hotrod.com/how-to/1006rc...ors-conversion

https://www.jalopyjournal.com/forum/...ights.1209713/

 

Here is a picture of the correct 68 T3 lens:

Reproduction T-3 Headlight Bulbs (Sealed Beam ), Small Bulb Kits, & Fuse/Flasher Sets for Classic & Muscle Cars

 

I installed these on my 1968 Olds 98. LED headlamps with DRL's (white) as well as directional indicator (amber). Got them from Amazon

 

Both LED and halogen lights are more efficient than the original T-3s. And, since the light output is limited by regulation, this means that your new lights will take less power than the T-3s.

But, the existing harness has quite a bit of voltage drop, made worse by age and corrosion of connectors. For LEDs, this will matter very little. But if you choose halogens, you may want to install two relays, one for high beams and one for low beams. Switch the relays on and off with the original headlight wiring. Power the relays through a circuit breaker direct from the battery.

By using relays you will get nearly double the light output from T-3s or halogens. Daniel Stern (referenced by 70sgeek in post 3) will have a kit to accomplish this.