Is this a good deal for a wiring harness?

The engine really doesn’t matter when it comes to the harness. You need to make sure the bulkhead is the same and all the plugs/connections.
Exactly.
so I figured it would work.
30 years ago when I started in this, I used to think like that.
So what I'm going to write next may come across as harsh, but its not meant that way.
You need to print a copy of the Chrysler wiring diagram from the shop manual. And then to be sure, compare the wiring to your cars.
I won't do the first for you and can't do the second for you.
It is work, but its not as hard as it seems.
I've already done a 'typical' main circuit and what your '76 main circuit may look like.
1 ->You need to verify that because its a slightly different and arguably better strategy.
2a ->Then you need to find the short.
2b -> Deciding on whether the '73 harness will work on your '76 will become a lot easier.