Is the vent open?
When the fluid heats up it expands. Between the reduced air volume, and the heated air, if the vent is plugged the pressure inside the trans will build up and blow the oil out the weakest link.
The filler plug is the level plug like RRR said. There is no good reason to run any more than that. If that amount of fluid cannot keep the trans cool, something is wrong.
When you had it apart;
1) did you notice the seal in the front retainer? Did you replace it, and if yes did you install it in the correct orientation? which is open side to the oil, and with the spring installed. What tool did you use?
2) did you notice that the case has a drain back channel from the seal side of the front bearing, and that the gasket has a matching cutout in that area.
3) the cluster-pin is a light press-fit into the front of the case. At the back, remember the half-moon key? it orients the pin and keeps it from spinning. But the press fit seals it. So the last half inch or so, should have taken a few hammer-blows to seat it.
4) the screws that keep the retainer on, from the factory are special. The holes are open into the case, so the special screws are made with a sealing ring on them and a serrated area designed so that the screw holds it's torque, and no washers are used. In the field, those screws are usually installed with a thread-sealant, cuz who knows how many times those screws have been re-used. If the screws have been replaced with standard screws and spring-loc washers used without sealant, well that is a guaranteed recipe for leakage.
5) did you notice, on the retainer, where the TO-bearing rides, the hole drilled in the bottom run, near the base, where the seal is? This is a drainage channel for oil leaking past the retainer-seal, so that it does not run into the clutch.
Laying on your back under the bellhousing, with the inspection-cover removed; you should be able to see that hole, and the trail of oil coming down the inside of the BH, from it.
6)Most Mopar manual transmissions that I have seen, had the vent in the tail, behind the back bearing, and with an internal baffle.
But, IIRC, some of those transmissions, were vented thru a hole in the top cover. The top gasked underneath it then, had to also have a hole, just not in the same place,lol. If yours is like that, then it better have the hole in the gasket. If you have to make one, I put it at the back, where the gears are spinning the slowest, and towards the passenger side, and NOT on top of first gear, which is acting like an oil-flinger any time the car is moving.
7) If the back of your car is way higher than the front ............................ give your head a shake,lol. The filler-plug assumes the car is close to level.
8) Oh yeah, I have seen retainers crack in the case-side of the seal, just below the seal area...... If you suspect something like that, I would pressure-test the trans thru the vent if you have one on the top of the tail. Try to keep the pressure under 2 psi, else you will blow the top cover. Lacking that vent, I would drill a hole in the fillerplug and press in a small brass pipe.
Good luck.
