I see absolutely no advantage to swapping the already installed and working 4 speed SM420 for a later SM465.
Both are 4 speeds, both are direct drive in high gear.
To swap the 465 I believe you need to do some machine work on the bearing retainer and or bellhousing bore. From memory the old one has 4-11/16" diameter and the newer transmission ls likely to be 5-1/8".
On the other hand the SM465 is a good transmission, but I think no better than what you already have in this case, and it's some trouble and expense to swap.
If you're only unhappy with the PTO then remove it, 6 bolts, make a cover or borrow one from the other transmission.
Grigg
