I would provide the comments on the 61 Z bar for 6 cylinder and 348.
The 409 Z bar has a one arm that is longer... I knew that 409's had a longer arm -348 probably did also ?
The only difference could be one of the arms on the Z bar, slightly longer. I have 6 or 7 - 1961 Z bars, they are all the same.
I have removed Z bars from 6 cylinder 61's and 62's on parts cars and in junk yards in Iowa, Nebraska and South Dakota and they are the same for 6 cylinder and 283/327 cars. Over the years I saw 100s of 61's and 62's in junk yards and cars friends and "I always looked at the clutch items" on the original cars. They fit v-8's including 409's & 348's and I have used them. The 61 uses a threaded lower rod with lots of adjustment and a fixed length upper rod. We installed a complete 1961 Chevy original clutch system/original bell housing and one of my 1961 Z bars in a 61 Chevy 409 Hardtop (the car was an original 348 TurboGlide Hardtop converted to a 409) and everything fit fine(adjustment was on the lower threaded rod).
All 1961 Clutch rods, arms and Z bars are different than the 1962's and 63's - rods, arms and Z bars.
Also 1961 z-bars have a stud on one arm(the upper one) and a hole in the lower arm.
62 Z bars have holes in the two arms for the upper and lower rods -the 1962 409 Z bar does have "one Z bar arm" that is longer but I have found that the available adjustment on the upper rod allows the use of the 6 cylinder and 283/327 Z bar on a 409 and a 348.
I have complete 61, 62, 63 and 64 Chevy clutch linkage parts that I have
had for many years. I just recently bought a good sized group of 61,62 and 63 clutch parts including Z bars.
Paul