Any help most appreciated in identifing transmissions. Casting #3925660, tailshaft casting#3857584.dated P8T04, Thanks in advance.

Date is December 4th, for the '68 model year. Casting #s are correct for the year, also, but no way to tell what model trans it is without looking at the guts, unfortunately. IF(big IF) it has never been rebuilt, then you can sometimes tell by the number of, or lack of rings on the input shaft. M20 would have 2 rings, M21 would have 1, and the M22 would have none....typically. This is NOT a sure-fire way to ID the ratio, but some think it is. Best way is to pull the shifter cover and count the gear teeth. While the M22 could have been ordered with the 427, it doesn't mean that all big blocks had the M22. The noise that a M22 makes is a good indication....they are extreeemly noisy!! 

Another way to determine the trans model is by the stamp pad numbers on the engine. Those numbers will tell what car, engine/trans combo the engine is for originally. If you can post the stamp pad numbers, we can narrow down the possibilities. 👍

Thanks for ur help. Found engine #T1129IL.     1118S407407

Tonawanda build, Nov 29. IL =L36, 390hp 427, Quadrajet carb, 4 speed trans. Base trans was the wide ratio Muncie, but the close ratio could be ordered.