Differential number code

[JJchamp48]

This is the number I found on my diff: 3896838. Can anyone tell me what I have?
According to option plate I have a 4:11 rear end. I am switching to 3:42 and trying to make sure I need the new carrier, before I get it.

Also, on transmission this is #: 8THM097H. This is supposed to be original tranny and powerglide.
Casting # on housing: 8663548; # on tailpiece: 8642547-1.

Can't seem to find any #'s on tranny that is in truck now.

I have a 68 Shortbed step original engine was a 6 cyl, switched over to 350 and soon to be 327.

[raycow]

Your casting number tells only that you have a 12 bolt truck rear. It says nothing about what ratio you have. If you want to find out what gears are in there without opening it up you can "count" the ratio.

Jack one rear wheel off the ground, put the transmission in neutral, and the parking brake off. Put matching marks on the pinion yoke and differential housing. Turn the wheel exactly TWO revolutions and count how many times the driveshaft turns, estimating any fractional turn as closely as possible. That number is your axle ratio.

If the wheel is difficult or impossible to turn, you may have posi. In that case, jack both rear wheels off the ground and turn either of them only ONE revolution. Count the driveshaft turns the same way as for an open rear.

If your ratio is 3.73 or numerically higher, you will need to replace the differential case, which some people call a carrier, in order to run 3.42 gears.

Sorry, but I can't help you identify the transmission.

Ray