Where can I purchase OEM engine wiring harness?

I doubt you will find a replacement harness for that year unless it is off of a salvage vehicle. What is the reason for replacement? Physical damage or? Personally I would only repair the parts that are needed or if you want all new I would go the custom route and make a new harness using one of the aftermarket units. That would also allow you to add or eliminate connections as needed. The main reason there are no OEM style replacements is because very few people would be staying with the stock harness and instead would be interested in engine or transmission swaps, replacing lighting and other electronics that either are different or nonexistent in the factory wiring. Plus the OEM wiring generally has a low count of fuses and to add things on you end up with running sub-harness blocks.
You can get close with some of the items that LMC Truck, Painless Wiring, Keep it clean, and a couple others sell and those can also be easily upgraded.

Thanks. I currently have the engine pulled out to be rebuilt, so I thought I'd look at the wiring at the same time. It's old (but probably still works), dirty, and not very easily identifiable.. which makes me nervous when putting the engine back in. Basically, I'd like something simple to help me, i.e.: the red wire goes here, the purple wire goes here, etc., but my wires are all black and dirty. :)

Yeah, they tend to get that way as the grease and heat works on them over time. I've had some luck using simple green on dirty wiring and not doing damage. Beyond that I would look at the sources above, the bulkhead connectors are available and with that and a supply of wire you could replace the harness one wire at a time. They did sell the full wiring diagrams as well that showed all of the optional wires for the various versions of those trucks. Those make things easier.

Can I buy one of these harnesses and modify it as needed to fit my '84 454? i.e. splice in wire to extend as needed, etc.?

https://www.classicparts.com/1981-Engine-Harness-454/productinfo/52-196/#
https://www.summitracing.com/parts/opg-11805

Could I buy one of these and just splice in extensions as needed?

https://swclassics.com/engine-harness-v8-w-electric-choke-fits-1981-87-chevy-and-gmc-truck/

Possibly, give them a call and see what they include in that harness. They likely make the harness to order and if you talk to them they may be able to make you the harness as a plug and play part that you wouldn't need to alter. But then it comes to what you want to do and you're back in custom harness territory again.

I'm trying to reply to my other thread on this subject, but it won't let me. I'm wondering if I can buy one of these engine harnesses and modify it as needed? Maybe splice in extra wire as needed, etc.?

https://www.summitracing.com/parts/opg-11805

https://www.classicparts.com/1981-Engine-Harness-454/productinfo/52-196/#

Saw your other message.You can use the one you posted earlier but call them to talk to them first. Give them the information on the vehicle and any additions you want to make and see what they say.

I was able to track down a wiring harness that should work. Some related electrical questions though: I'd like to replace the stock battery cables with some bigger gauge cables that attach to posts rather than the screw-in type stock cables. I'm thinking 02 gauge, will that be okay? Can you tell me what the little pigtail wires are for on each of the stock cables? And, can you also tell me where the red cable attaches to coming off of the battery?

Bigger cables are not a bad idea but not really needed unless you are seeing voltage issues when starting. If possible just buy premade cables and install those, the clamp styles are available in many places. The smaller wires feed different things depending on the options. On the ground side they connect to the chassis. The battery feed should connect to the starter motor main terminal, with that having a jumper connection to the junction connector on the firewall and from that to the fuse panel. The attached is the wiring for the optional dual battery system. The relay acts to allow charging when the engine is running but disconnects the battery for starting. It basically splits the system so the main battery powers the starter and the auxiliary runs lights, winches and other loads.