The frame was from a 59 Belair and 6cyl Absolutely perfect appearing frame.
But, I just looked in the service manual and there is no dimension for between the mounts. I did check the dimension from outside to outside the rails before I started building it---was right at the target my book tells me it is supposed to be---in fact all cross checking came within a 1/4 inch(near as I could measure without fancy tools). But I have noticed a LOT of these X-frames in yards that have no shims in them. (have 2 in my yard that only have 1/16" at each location.) This car was never designed for a Mark motor, so I knew that I would have issues with that right out of the gate. Tubular arms gave me almost 3/4" clearance on that arm----only issue was with the left side--On the right--the exhaust port is behind the front upper bushing.