64, I see Hughes supplied the heads. Do you have paperwork that says what the installed heights of the springs are? I'm not a Magnum guy but perhaps Brian can help me here... The Magnum installed height is typically shorter unless the valves are longer. Around 1.600" with stock valves. The 1122 spring at an installed height of 1.830 has about 335lbs at .600 lift according to Hughe's site. The dual spring they recommend (1111) is installed at 1.80 and has the same 333lbs at .600 lift. If they set the spring heights up for even the longer Chevy valves (.100") you're still way short for that spring. Meaning the pressures will be WAY higher. I doubt it would coil bind, but the pressures may be too high for the cam. If you verified all you said (and I don't doubt that part) then there's got to be an issue with soemthing you couldn't verify.
On cleaning teh block... Yeah, take it out, strip it to bare, wash it with soap and brushes, and reassemble the whole thing. Iron dust went everywhere and not cleaning it is a fast way to wipe out everything else down the road. Oil changes and flushes don't cut it when it's all through it. I'm doing the same with an engine that ate a timing cover now. It sucks.