main problem is 60 years of abuse, it's been driven away with the PTO still engaged way too many times .... when cold things are a bit more snugged up - as the gear oil heats up things can move around more
the SM465 [1968+] will be smoother than the SM420, but any used trans, especially one that's had a long life with a PTO could be noisey .... either newer trans might bolt to your bellhouse, but either may have different input and/or output shafts than your current one
Bill
