You make no mention of what your combination is... Small block? Big block? Slantie? 904, 727? What vintage engine/trans.?
First thing that comes to mind is that you're trying to mate a post '68 transmission to a '67 and earlier engine or crankshaft- the convertor noses and crankshaft registers changed dimensions in approximately the '67/'68 model year and you could be trying to force a large diameter convertor nose into a small diameter crank register bore... but I don't know.
More info please.