I replaced the lean burn slant six in my '84 D100 with a 440 more than thirty years ago. The wiring of the engine harness has become brittle over time. Of course the harness is no longer produced. Does anyone know where I might find a decent used harness. My guess would be any old harness has the same problems as mine. Any suggestions on what to do?

I got lucky at a local junkyard when I had to redo my 85.

all jokes aside, if your connector [plastic] pieces are in good condition, a handful of quality brass connectors, a good crimping tool or two, and a few spools of the correct color wire will easily duplicate a harness for a fraction of the cost of a reproduction harness.
it's not as hard as one would think, just take your time and be patient.
get extra terminal ends and practice on several chunks of junk wire until you get the crimping process down.

Great looking truck.

What I would look to do if I was in a similar situation and wanted to retain a factory harness is to get an original harness and use it as a pattern for copying and making a new one. That way you can take your time to make it correct and you can keep driving your truck until you're ready to make the swap. This would also reduce the time your truck is down.

Yup.
The original harness off of my 85 was the worst butchered harness I'd ever seen.
The alternator wire was twisted and taped, changed colors and gauge size 3x in the length of the/6 valve cover, with bare strands hanging out of the tape job. One dangling within an inch of the fuel line to the carb. A lot of house wire, a lot of fried fusible links.... I went to the junkyard and got another, and undid all the original tape and went thru the whole thing closely/ I found a few suspect areas which I fixed with crimps solder and heat shrink. I also did away with every fusible link, instead I found an under hood fuse box from "something else" and wired everything that was previously run thru a fusible link to/thru that.
Its beyond me, what they were thinking when they decided to wire everything thru softer wire 3 gauge sizes smaller than the parent circuit, and the way they crimped it into the harness to where it couldn't be properly replaced if and when it cooked.

So your 84 harness has already been modified for a big block. If your bulkhead is good then just buy wire and make a new one. Its the engine harness so there is only about 8 wires.

i have been using marine grade heat shrink with the "glue" when heat shrink is required, for quite a few years now.
i highly recommend it.

Yup agreed when it can be found.
The normal stuff from the hardware store sucks

amazom has it. just one example below.

https://www.amazon.com/Wirefy-200-Heat-Shrink-Tubing/dp/B089D82FLG/ref=sr_1_5?

It looks like the answer is: Make One. So, I'll give that a shot. Thanks!!

making one is not as hard as it is intimidating.
use quality wire [automotive grade] and quality terminals, crimped with the proper crimping tool, and take your time.
you will do a good job.

Where can I find the correct terminals?

Digikey.

digikey indeed !


Digikey
https://www.digikey.com