The upper bolt holes in the bellhousing/block are a different pattern between the six bangers and the V8... the sixes are closer together. There may be other differences as well. So, if I understood your question, a Mummert T-5 conversion for your 223 cannot be used later on your y-block.
Cheers... Brian
Cylinder Index = 84
Current Experiment = `57 Tbird, Grand Rapids, Michigan