I installed a Desoto fluid drive engine in my non fluid drive 1948 Plymouth. I used my Plymouth bell housng and flywheel. Due to the mating flange on the end of the crankshaft and the different bell housing I had to adjust (remove about 3/16th") the starter mating surface. I carefully measured this on the bench and did this prior to installing this combination in my car. This may be part of your problem but I do not know what combination of parts you used.
