Conversion Wont Fit

I was trying to put disc brakes on my 1969 El Camino and I was told by a mechanic that the easiest way to this and save some money was to get the caliper, rotor, upper/lower control arm, and the coil-over-spring assembly from a 1991 Chevrolet Caprice because it would mount up and the only thing that doesn't fit is the upper control arm mounting bracket because the holes are to close together and i was wondering if anyone had a quick fix idea?

slot the holes and use thick washers?

I'd lookup Speedway Motors or a co. called AFCO either one should have a weld on upper control arm mount that would work just fine. The roundy rounders use em all the time. Or why not just re use the Elkys upper control arms ???

By "mounting bracket" are you referring to the crosshaft on the upper arm???