Rearend I.D. help?????

[JaggedEdge]

So I bought this toy about three years ago.
It's a 66 Chevy short wide 4x4 with a small block 400 and an automatic.
After raggin the hell out of it, metal worms have finally made me tear it down to rebuild. (the cab floors were gone and the cab was settling)
I am trying to figure out what the rearend came out of.
It is NOT stock to this truck.
It is a GM 12 bolt, mounted on a ridiculous 7 leaf high arch spring pack. The spring perches have been remounted I am assuming to correct the pinion angle. (It also has 4" lift blocks.)
They have also been moved outboard one inch on both sides.
The previous owner (who did not build the truck) told me when I bought it the running gear was out of a '77 blazer. The frame numbers are from a '66.
I have searched on the web but can't find anything definitive.

So... anyone "in the know" who can help me verify what it came out of , I would really appreciate it... or maybe you could point me in the direction of where I could find it.

3896839 N -- casting number on the housing
E 4 7 -- date code on the housing
KAH 126W71 -- axle code on the passenger tube

Don't know what the gear ratio is as I have never had the cover off of it.
If anyone could help??????
Also forgot to mention the axles are standard chev 6 lug.