If the 4 speed in your '57 is original, it's an sm420. Those bellhousings have a 4 11/16 bore for the front bearing retainer.
The sm465 that came along in '73 had the bigger 5 1/8 front bearing retainer.
All the three speeds I've seen had the same 4 11/16 front bearing retainer as the sm420.
When I swapped my 3 speed for an sm420, it was bolt in. No mods to the bellhousing were needed
Thanks! That's a huge help!
As for the old motor, I don't see any cracks in the block. My buddy's dad is interested in my 261, but I haven't really talked much about it.
