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I went with a 302 and AOD in my '55. Its been some years but my recollection is that I mounted up the radiator frame, then connected the engine and trans and held them in place hooked to cherry picker. Once I was satisfied with the location, I marked the locations for both the engine mounts and the trans crossmember. I think I got both from LMC.
Maybe someone will have some measurements for you, but I would think you would need to mock-up the engine and trans because you need to make sure you have the proper 3 deg. down angle on the engine, which effects the trans crossmember and engine crossmember heights, make sure the rad will clear the fan, etc.
I've got both cross members from mid fifty. Seem pretty simple just checking to see if anyone had the measurements. I didn't want to goof on my first attempt.
The above mentioned methods would be best, as measurement will be useless if you're not using exactly the same set-ups as other people. Since there are different radiators, fans, pulleys, etc., you need to use your own measurements. Keep the fan blade within an inch of the radiator surface, and keep it centered between the frame rails. The angle of the engine isn't exactly specific, but the mentioned 3* is going to be pretty close to a normal amount. That should give you a nearly level carb mount area, and tranny pan.