Looking at the pictures, you have an Eaton 17201 or 17220 (23,000) rear end. I just changed my ring and pinion on mine from 5.29 to 4.56. My truck has a 4-53T Detroit Diesel which is governed at 2,500. With this change I now have a road speed of 67 MPH. These Eaton's are heavy duty rear ends usually found in larger trucks. I doubt anything a 366 could dish out would do any damage to this rear. The way to tell you have the heavy rear end is the axles are held in with 12 bolts (glove box will show the Eaton 17201 or 17220). Here is a link to my rear end ring and pinion replacement.
The 17220 is a later version with 32 splines, vs the earlier 16 splines, and a slightly different pinion carrier assembly.
http://67-72chevytrucks.com/vboard/s...d.php?t=729084