What you provided is a good start but with out being there or being able to see pictures we need more details.
You said the brake lights work but do the tail lights work? That may change some of my theory. If the tail lights don't work, you may not be getting 12v from the alternator. Possibly due to a blown fusable link or just a bad alt.
There's a lot here and I've changed several things as I learned more in researching it but I left it all incase something might help. There are a million ways to go about this. Read through it and hopefully something will point you in the right direction.
First thing, check for 12vdc at the two red wires in the bulkhead connector. They are coming from the alternator and I believe they should both have 12v all the time. Now because I suspect a missing ground, first put your negative lead on the neg batt terminal itself and the red lead on the red wire in the bulkhead connector for this check. Then if you do have 12v, move the black meter lead to the sheetmetal on the firewall. If you don't have 12v now, you're missing a ground wire. Probably the large bare braided wire that goes from the block to the firewall.
Three of the things you mentioned, headlights, interior lights and horn should all have power all the times so, for those things, you can eliminate the ignition switch or connection to it as a source of the problem. You could have multiple causes but again, it sounds like you have a bad or missing ground in the engine compartment.
The red(pos) battery cable should go over to the alternator. Then another red wire, possibly two, should come off the alt and go to the bulkhead connector on the firewall. This wire may (probably) has a fusable link in it which could be burnt. The red wire may change color to black or green where the fusable link is. Not because it's burnt, that's just how the schematic is drawn. The schematic is unclear as to whether there is one or two fusable links and red wires going to the bulkhead connector. That easily checked just by looking at your connector. I believe this is the only "main fuse" in the engine compartment. One red wire goes to the corner pin on the bulkhead connector near one of the screws. The other goes one row down and 4 pins away from the first red wire. These appear to be the only red wires in the connector. These are what bring power into the cabin. One of them splits once inside the cabin. One leg goes to the fuse box to the fuse for the tail lights. You said the brake lights work but do the tail lights work? If so, that wire must have power. However, one of the legs that splits from that same wire goes to the horn relay which you said didn't work. This is why I think bad ground up front. The other red wire goes directly to the head light switch. The head lights ground to the front sheet metal which, if not connected to the negative battery terminal via the engine block to firewall ground wire, won't work.
The blower motor uses a resistor pack to vary the speed of the motor. It is mounted on the a/c box if you have a/c. I suppose it's on the firewall if you don't. Anyway, those resistors can open causing the blower to not work on high speed. It should always be running at low speed when the key is in and on. This is for positive air pressure in the cabin while the engine is running.
If you removed the engine, you may have forgotten to reinstall the braided ground wire the goes from the engine block to the firewall. I believe there is another bare braided ground that goes from the trans to the frame on the pass side. These are very important.
Does the car run? I am wondering if the plugs on the ignition switch attached to the steering column have been plugged in good.
If the car's interior has been taken apart, make sure that you have reconnected the dash/instrument panel ground wire located near the driver side courtesy light bulb. It's just a 12 gauge or so black wire that attaches to the metal dash via a screw into the dash through the eyelet on the wire.
If none of this helps and you still have the original harness, lay it out and go through it wire by wire and verify what each does and see if there is a wire that is needed but missing from the new harness.
Good luck,
