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Well, I stripped it down and am very surprised that the bike didn't fall into pieces while being on the road. There are rear wheel spacers missing, loose hardware on the front wheel/axle, hastily mounted shocks, etc.

I'm going to start cleaning up the poorly de-tabbed frame and thought about fabricating an upper mount for a mono/LC setup. I have heard about the swap, but I haven't found any pros-cons. Anyone have any input? It seemed like a fun project to do with the extra time I have in the workshop.

pretty sure its of an 80s ninja, rear caliper of all things.

Yep. That's the one. I found it this morning.

Rear brake is a stock early 400 that should have that flat torque arm below the swingarm.
It's possible that the swingarm is a later model that had a top mounted torque arm. 

Front appears to be a later single piston sliding caliper on stock RD forks.

Bit of a mix and match.

If I wanted a rear disc brake on s RD350, I would definitely look for something smaller and lighter. A 220 mm disc, 4 mm thick, and a small, lightweight caliper with 25-34 mm bore, most likely Brembo. That stock RD400 setup is totally overkill in so many ways.

I agree. It's not going back on.

That seat looks like a torture device.

There are a lot of interesting things on that bike. My favorite was that instead of running a regular battery, they chose to use a 2-pack of 6v lantern batteries (still in the packaging) run in series with alligator clips and 20g wire.

I think you would have to search for quite a while, to find a better caliper than the OEM 2 x 48 mm piston caliper. They were even used on TZ250/350/500 roadracers.  The alloy reproduction one from EC is great, I have one on my 350.
If you want something else than OEM/ reproduction, while still being a 2 piston setup, then 41 mm Lockheed or the rare 48 mm Brembo are the main alternatives IMHO.
On the other hand, if yoy want the best and lightest available brake, then a single 300-320 mm flaoting disc, and a suitable 4 piston caliper plus an adapter plate is the best option.
I would not recommend adding a second front disc to any RD, unless for an all out roadracer. The added unsprung weight will hurt handling and suspension, and the added braking is really not needed.
(I binned the heavy and weak OEM dual disc setup on my 77 XS650, and replaced it with a single 320 mm ISR disc, a Brembo 4 piston caliper, and a home made adapter plate. Saved around 4 kg of unsprung weight, and now have a much better brake)

That's why I was wondering why they used that crazy setup. It's paired with a newer sport bike master cylinder. It's a crazy build, but it looks like I'll be ditching all of it and going back to stock components.

[Front appears to be a later single piston sliding caliper on stock RD forks]

Rear brake is a stock early 400 that should have that flat torque arm below the swingarm.
It's possible that the swingarm is a later model that had a top mounted torque arm. 

Front appears to be a later single piston sliding caliper on stock RD forks.

Bit of a mix and match.

The only thing I can find that matches the design is a modern caliper for scooters. It's a really strange setup and I'm wondering what the advantage was over the stock caliper or even a twin disc setup.

As to the potentially pinched fork. Just loosen the two M8 nuts at the bottom of the LH fork leg a turn or two, depress the fork a couple of times, and re- tighten the nuts. If nothing is bent, then everything should be ok.
Please note that the hole in the LH end of the front wheel axle may end up partly inside the axle clamp/fork leg . This is how all RD350s are, at least those I have seen. To pull the axle, using a Phillips screwdriver or similar, it just needs to be tapped with a mallet from the other side first.
The location of that hole must have been some kind of design or manufacturing screwup on the first place.

Sounds good. I was going to measure the next time I was with the bike, but it was in my head so I though I'd ask. I remembered it looking a little off when I left it and it's stuck with me since.

Rear brake is a stock early 400 that should have that flat torque arm below the swingarm.
It's possible that the swingarm is a later model that had a top mounted torque arm. 

Front appears to be a later single piston sliding caliper on stock RD forks.

Bit of a mix and match.

Perfect. That's what I was looking for. How do I identify a stock front wheel?

some weird brake mods. they look like old ones at that.

welcome.

Yep, and a lot of the bike was being held together with zip ties and hot glue. I really only wanted the frame, but the roller came with the title and I picked it up really cheap.

Hello everyone,

This is my first post and my first RD! I picked this thing up after "needing" a new project to take on. It's a 73' that the original builder made some interesting modifications to and I was wondering if anyone could shed some light on what he did with the front and rear brakes. Additionally, I was hoping that someone could tell me the correct width of the front forks or front wheel axle. The forks look a bit pinched at the bottom. Anyway, I'm starting to plan the build and want to know what I'll be keeping and what's going to a new home or in the bin.

Happy to be here and thank you for the help!