With a pair of 8" dial calipers measure the big end bore inside diameter, then measure the wristpin bore inside diameter, write the diameters down. Then measure the length from the bottom of the wristpin bore down to the top of the big end bore, write down the length. Divide the wristpin bore by 2 write it down. Divide the big end bore by 2, again write this number down. Add the length plus the two divided bore diameters together and that will give you the total center to center length
example: Stock buick rod wristpin bore = .940 divided by 2 = 470
Stock big end bore = 2.250 divided by 2 = 1.125
Length from bottom of pin bore to top of big end bore = 4.365