first you need to know which muncie you have. input shaft with no lines is a 1.91 2nd, 1 line - 1.64 2nd, 2 lines - 1.88 2nd. a final between 6.00 and 6.30.
example -
1.91x3.25=6.21   i think you will be looking for either a 3.25 or so for the 1.88 and 1.91 or a 3.89 for the 1.64,

Ya I have a powerglide with a direct drive coupler and a 5.67 gear now. Hows that late model coming.