I talked to my mechanic about the weakness of a W5* box a while back, as I'm using one in my KE70 build. He's seen many broken behind V8's, which he puts it down to end float in the shafts causing the angular teeth to bind.
They've run 600+ HP through them with straight cut gears with heavily modified chev V8's and got years out of the box under race conditions without the porche roller bearing mod that many use,
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