[quoteI see it mentioned boring out the old or welding new spindle on to accomodate 35 spline axle shafts.]
Not sure where you heard that unless you are dealing with a really early axle version. I went through three housings on my 75 Highboy to make one....loooooong story.
Anyway, the original had 16 spline shafts. Other two were either 31/35 I can't remember. The spindles and hubs all looked the same and I know I threw the axles side by side...only difference was the spline count. I ended up using the ring and pinion from my original housing, the carrier and spiders and brake parts from a dana 61, and put it all into a late 70s/early 80s housing after cutting and repositioning the axle perches to match my original High Boy axle. Lot of part swapping. If my memory was better I could tell you 100% for sure but at the time that happened I had a lot of stuff going on. Either way I drove the snot out of it and zero issues.
16 spline axles are a lot stronger than people want to admit. Just as an FYI lot of big trucks run less splines, but bigger cuts....go look at a 2 1/2 or 5 ton military axle.....looks a lot like what you have, only bigger.
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I looked at a 77 Highboy and a couple of bump side 60's that had the spring perches in the same location as my 65. Interesting thing was there was an 1 1/2" difference between backing plates, mine being the narrowest. Seems like the 77 was the widest.