I did forget to answer your original question. The pin length is identical.
The connecting rod(s) are done a bit differently on each loco. While both locos have the three rear driver axles gear driven, the crank pins are used on different drivers.
Mikado only has the side rods pinned at the 1st and 3rd (main) drivers.
D51 has the side rods pinned at the 1st, 3rd (main), and 4th driver. But since the 4th (rear) driver is gear-driven, that pin is just cosmetic.
The axle spacing does appear to be the same on both locos, so the the rods and valve gear should be easily interchangeable. The different valve gear on the D51 is probably a bonus, since it depicts a different type of valve gear which might have been used on some American 2-8-2s.