'74 Challenger came with the wonderful seat belt interlock device in them. I've been grounding this interlock to allow everything to work properly without having the belts latched. With their age and failure risk, I'm now at the point of replacing the harnesses in the car and am wondering if simply replacing it with a 73 or 72 harness allows everything else to be the same except the interlock circuits. Also, do all the interior and under hood harnesses need to be replaced at the same time or can I start under the hood and then replace the interior harness later?

While we are at it, what are thoughts about ditching the stock replacement style altogether for an upgraded set up like American Autowire? The additional circuits and the ATC fuses seem like a nice change from OEM. On the other hand, a matching OEM harness allows me to follow factory wire diagrams, color codes, and trouble shooting.

Pros or cons of the above?

How stock/original is the car, or where it's headed, so to speak ?
What's the condition of the various harnesses at present ?
The interlock - whether by grounding or disconnecting any mechanical bits - shouldn't present an issue, as far as I know.
I've rewired a few vehicle with American Autowire generic harnesses; no problems, and would buy from them again.
As far as doing all of the harnesses at the same time, that depends more on condition and/or future plans, IMO.

with all including the American Autowire.

Current condition on the engine bay harnesses is what you'd expect for 50 year old wiring - present and mostly functional with drying and cracking on those wires nearest heat sources that have had some repairs and taping over its lifetime. All forward lights work without issue. At some point it did have something short out in the brake light switch circuit under the dash as those wires show obvious heat damage but have not burned through. The rear light harness needs attention as the taillights do not work. This is probably related to the brake light circuit issue, but I haven't dug into it yet. It is a factory a/c car so it has that extra harness under the hood, but it fortunately does not have the usual bulkhead connector burns on the main power lead going into the car.

Electrically the car is mostly, but not entirely, stock. I have added a single wire alternator with its lead going to the battery, and it has an electric fuel pump, which is switched on with a keyed circuit in the fuse block, powered off the battery relay, and using its own standalone relay and fuse. It already has electronic ignition wired into the factory harness.

For future plans, nothing too exciting. I don't care to add power windows, locks, electric seats, massive stereo equipment or things like that. A change to a tilt column may be nice, but I'm not sure. I haven't driven this car around enough to know if that's something I'd like to do, but most seem to use GM based wiring. I've got a clutch fan now so doubt I'd ever add electrics, but you never know. Compatibility with the electric fuel pump may be nice, but not critical since i already have that set up outside the OEM harness. I may also want to upgrade the a/c to a vintage air system in the future, but I assume they build those to be compatible with or work around the stock harnesses. I would like to eliminate the ammeter just because I've seen those circuits cause so many issues in other Mopars, although this one seems to have escaped that damage so far.

American Autowire does make an E body specific harness that looks pretty nice and I have seen quite a few positive reviews on their products. I may pick one up for my next race car project just for the simplicity of their layout.

Any replacement harnesses you use I would check the crimps before installing. I had a replacement harness with some bad crimps some years back.

I just unplugged the connector under the seats on my 74 Challenger to disable the interlock. That has worked since the car was new. That was one of the first mods I did to it.

You don’t need to ground or unplug anything to bypass the interlock system. You can simply jump out the two yellow wires at the reset button plug. There are some other methods, but that’s the easier one. Swap out the two yellow wires at bulkhead conector works too.

That will keep operative the warning systems (buzzer, lights) but still will be able to crank up the engine no matter what.

The next easier solution is get the orange control box, replacing the blue if auto or white if manual. The orange box is a non interlocked box just a warning signals control. Tipically used on Canadian/Export models.

Based on my 71/74 B body experience, there is no a STRAIGHT interchange on harnesses except for the rear section (body and tail lights). Pretty sure E body’s should be similar.

72/73 get also seatbelt warning system, relay controlled. Seat sensors are the same to 74s, but not seatbelts sensors, while on 74 sensors are on females, 72/73 are on the retractors.

74s use a single buzzer for everything controlled by the control box, while earliers use a single buzzer for every function. Even the horn relay is diff between 74 and earliers.

On B bodies there also several differences on the schematic distribution at bulkhead. Several wires were swapped around along all the 71/74 production. Several harness changed also, (A/C for example) I can post details on this if wished. E bodies could be similar.