1964 Valiant Slant 6 to 318 Conversion ?

Did you refill the trans with the motor running in N? (Park wont fill the convertor). A dead stick pour wont fill the convertor and may show full on the stick but as soon as you start it up and put it in N, the convertor will fill and swallow much of that new trans fluid. Now if you check it, it will look empty again and slip or wont engage at all. Keep it in N and keep adding until the stick says F or at least Add. cycle the trans through the gears a few times and let idle in N again for a few min. check a 3rd time as now the valve body will be full as well as the cooler. I poured 9 quarts into a C4 when it was not running and 3 of them ended up on the floor like an expanding pool of blood under the car. The C4 has a vent on top that it came out of as the convertor didnt take any of its 'full capacity' fill. Most 727/904's wont fill in Park so its common to start a cold Mopar, and put it in N to let it warm up. That way when you drop it into D or R, you wont have a lazy engagement into gear.