I find it odd that you broke a shaft on the road. When I had that axle, I was on 37s and drove it frequently on the road with no problems. Also, I highly suggest you measure the spindles to ensure there is enough material for boring them. Because that came out of a 73 Dodge W300, it may not have large enough spindles. I never measured them when I had it since I already had another 60 to go in its place.
I was surprised it broke too. I think it was just turning a little too sharp out of the stop sign it was less than a 90 deg turn, so pretty sharp I should of known better. I had put plenty of miles on it too and it had been great, just a little chirpy while parking.
The spindles are already around 1.5" ID (quick tape measure measurement I took to figure out PVC pipe size for magnet) and from what I have gathered they need to be 1.56" ID to hold the 35 spline. So that basically 1/32" of material needs to be removed from all around the inside.
I do need to get a precise measurement and I will be doing that this weekend. I may not need to bore it all. Fingers crossed.