I have a Muncie four speed that isn't listed in my interchange book. It was supposed to be from a Chevy but no listing up to 1970, so I assume it's a later box. The #3925660 is on the upper right side and the #P9E27A is vertical on the right rear of case. What's the ratio and what's it good for, small block or big block? I'm pretty sure it's an M-21 because the cluster shaft is one inch and it has a 10 spline input with two rings on it. Thanks, OJ o No: