It’s gear oil, smells bad and pretty thick. Pretty funny story. I love hearing stories about people’s 400 motors. Definitely good motivation to one day get one for myself.
If that leak is around the bell housing then I wonder if the trans front seal is leaking. It’s also possible for a rubber seal to wear a groove in the mainshaft and that can cause leaks even with a new seal. It will get sorted out; no need to panic yet. Main thing; keep that gear oil full until this is repaired.
Glad y’all liked the Cyclone story. Guess we should have considered ourselves lucky the guy didn’t pull a gun since he was an off duty trooper and likely carrying. Back in the day around here the cops were often the biggest scofflaws of all.
One thing that Cyclone had though was looks. It made my buddy’s pale Yellow base Firebird with no PB, PS, or AC look pretty drab. The Mercury was in Grabber Lime Green with Mirage stripes and was drop dead gorgeous. Too bad it wasn’t faster… 
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It’s kind of odd that it’s leaking just sitting there, is it parked on an incline?
Unless you count the very slight incline from the way the garage is built, no and that incline would make it leak from the rear rather than the front if anything. I’m pretty confused too but I’m getting my jack back today so maybe I’ll get some clues later
Is it possible that you did not put it evenly all around causing the leak? or missed a spot? when you put the new shaft seal in, did it go in tight or easily?
Or cure to long before assembly ?
I suspect the dual 4bbl were to blame, too many CFMs for that engine.
Which shaft seals? The seals for the shifter shafts felt tight and I had to tap them in with a socket. I’ve always been told to do that with seals but maybe that was wrong too. People are mentioning a main seal up front or something similar although I don’t think there is one.
As it_s-me said I also didn’t give the sealer a lot of time to cure before putting everything together. What’s weird about that though is none of the deal I replaced are really below the fill level, except for possibly the bearing retainer gasket.
Could I have overfilled it? I thought you are supposed to fill it until it drips out the fill hole but maybe that’s wrong as well?
Fill until it drips out is the correct level. Apparently they do not have a front seal. It appears to use a gear oil slinger which would be similar to the oil slinger used on the old VW air cooled Beetles in their engines.
The VWs have a rear (actually front behind the flywheel) seal but on the front of the engine (at the tail end) they use nothing but an oil slinger. No seal at all.
At this point all I can suggest would be to jack it up (safely), clean the underside of the trans, and wait for a leak to appear. The possibilities should be very few unless somehow the case was cracked during the install.
Not year/model appropriate but oil slinger is 8.
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Yes, although I’m not sure on every exact part but looks pretty much the same as the diagram. And to ok4450 I’ll do that soon enough and hopefully figure this out. Something that worries me is that I’ve read they have a countershaft a bit below the input. There’s no seal for it but instead just has a tapered fit into the case that can wear. Maybe I disturbed it somehow and now it’s leaking.
The main thing at this point until this problem is sorted out is to keep an eye on the gear oil level.
About 20 years ago a neighbor had a manual trans Ford Probe. Someone had changed the gear oil and overtightened the drain plug. That cracked the aluminum case around the plug. A tiny 1/4 inch long crack that led to a very slow drip.
I warned him that what he should do is drain the oil, clean that crack out, and fill it with JB Weld, RTV, anything. Two weeks later the transmission gave up and busted the case due to lack of gear oil.
Then he had to pay me to swap the trans out…vastly more expensive.
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If the gear oil is leaking with the transmission just sitting unused, my guess is the leak is where two case parts meet up on a vertically oriented seam; i.e. the seam runs along the bottom of the trans. Good advice above to clean it thoroughly, then inspect on a daily basis, should be able to quickly narrow down the leak location. I expect this will be simple to solve, other than having to remove the transmission presuming it is currently installed. Did you use gaskets & sealer, or only sealer on the vertical seams?