727 trans won't seat-up on 440

I have an unknown make 3200rpm stall converter, flex plate from a '77 400bb and the engine is a '78 motor-home 440. I've seated and reseated the converter, but it sticks out too far. The bolt holes in the converter are roughly level with the mounting face of the bell-housing. So, if I put the trans up to the engine, there's about a 1/2" gap from the bell-housing and the engine. The converter and flex-plate will be touching though and I can't get it to close up the gap at all. Do I have a bad mix of parts, doing something wrong or just overlooking a small detail?