He kept his existing 3/4" “L-shaped” spindles on the tractor and just bored out the Northern Tool spindle to fit over it.
The bored out spindle slips over your existing spindle and is held in place by the cotter pin at the end running through both spindles.
Using the cotter pin allows you to upgrade to the tapered roller bearings and a 4 bolt hub without changing your current “L-shaped” spindle configuration
Stan C, in Medford, OR., used another technique to upgrade the “L-shaped” spindles on his 1968 Gilson 12 HP.
He reused the stock steering arms and incorporated them into a completely new "C" shaped spindle carrier.