Help to find the center line

[1958]

I need some info on how to find the center line on my 58 truck it has the old front mounts and we are installing side mounts when we got the truck it had no engine or tranny so just wondering what to do. Also when I went shopping for a tranny cross member they had two or three different kinds. one with a 4 inch drop down and one that was 6 inches and one that was 8 inches never new there was that many to choose from so which one is right for a turbo 350. Thanks

Chris,