I just picked up a muncie and I'm trying to figure out what it is. Any help is appreciated.

Pass side # P1117
Drivers side 6J155290
case 3885010
tail 3857584GM

Thanks

P1117 Nov 17, 1965 assembly date (1966 production year)
6J155290:
6 - 1966
J - Janesville assembly plant - Full size Chevy
155290 - 55290th vehicle produced at that assembly plant

3885010 - 1966/67 Muncie case casting number
3857584 - Muncie extension housing used mid 1965 through 1970

Is it an M20 then? I assume so.

Sound like you might need to count the teeth on the inside of the input shaft. Also might be able to tell by the number of rings on the outside spline of the input shaft.

Case casting numbers, relating production years and ratios available

Casting: 3831704 Years: 1963 Only Ratios: M20 2.56 1st , M21 2.20 1st
Casting: 3851325 Years: 1964 - 1965 Ratios: M20 2.56 1st , M21 2.20 1st
Casting: 3885010 Years: 1965 - 1967 Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st
Casting: 3925660 Years: 1968 - 1970 Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st
Casting: 3925661 Years: 1970 - 1974 Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st

Ratios of the M20 , M21 , M22

Year 1963-1965 Type: M20 Rings: None Ratio: 2.56 / 1.91 / 1.48 / 1.00 / 3.16
Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Year 1967-1974 Type: M22 Rings: None Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27

Input Shaft Tooth and Spline Count Related to Year

Year 1963-1965 Type: M20 Rings: None Spline: 10 Tooth Count: 24
Year 1966-1970 Type: M20 Rings: Two Spline: 10 Tooth Count: 21
Year 1970-1974 Type: M20 Rings: Two Spline: 26 Tooth Count: 21
Year 1963-1970 Type: M21 Rings: One Spline: 10 Tooth Count: 26
Year 1970-1974 Type: M21 Rings: One Spline: 26 Tooth Count: 26
Year 1967-1970 Type: M22 Rings: None Spline: 10 Tooth Count: 26
Year 1969-1974 Type: M22 Rings: None Spline: 26 Tooth Count: 26