JTR sells those, part # DAT-110 : https://shell7.tdl.com/~jags/Pages/Parts_DAT200.html
"The driveshaft flange bolts to the Z Car differential and uses a Dana #1310 U-joint, which was commonly used on most Chevrolets in the 1960s and through the early 1970s. The pilot diameter on the flange is 2.25 inches in diameter. All 1970-1978 Z Cars came with the 2.25" pilot diameter. Later (ZX) cars came with either a 2.25" or 2.00" pilot diameter. If the differential has the 2.00" pilot, the pinion flange will need to be changed to one with a 2.25" pilot."