Need someone with '68/'69 A-body rad experience.....

At 10mm's of difference over all, that's only 0.39" for us Yanks. The holes are slotted/oval/elliptical. I bet a rat tail would get it to fit, if even needed at all.
The dimensions look to be on center of the slot. Might see if a vendor will measure one for you or if someone here has one to measure.