Axle center line difference 1953 3100/3600

[1953]

Ok so im installing the front ifs speedway motors crossmember on my 53 3600, they have great instructions on the 1/2 ton installation but dont cover the 3600 3/4 ton, they are very identical and my question is what is the difference in the axle center line on the 3100 and 3600, and is finding the centerline on the 3600 exact same as finding it on the 3100 approx 18 1/8 from the front spring shackle hole? The cross member i have is speedway motors bolt in for 1/2 ton and light duty 3/4 ton.