"Any chance I already have either the adapter or the u-joint, considering the seized 235 that's in the truck now?"
3-speed u-joint and 4-speed u-joint are different (you need the 4-speed forward yoke and the 3-speed rear yoke).
The torque-tube adapter (rear cover) on the rear of the SM420 is an SM420 specific part.
Also, in order to bolt up to my 261, do I need to look for an SM420 from a particular era or with a particular feature? (# of splines, etc.)
Wait to hear from others on this one. I think that any year SM420 will work.
You would use the clutch housing and clutch assembly (including throw out bearing) from your 1950. Use a new pilot bushing in the rear on the driveshaft.
Your speedometer will be off unless the transmission has the drive and driven speedometer gears for a 4.11 rear end (try to buy a set in the Swap Meet).
