You might try some thicker valve cover gaskets. They make them. Seems weird the hold downs are what's hitting. Are they hitting the stands for the PCV baffle or something??
I had the mopar perf. covers hit a set of pro comp hold downs, u can machine / file the upper corners of the end hold downs , on a taper to not weaken them------- no biggie !
Had the same problem with hold-down blocks. The early stamped 273 Commando covers cleared nicely; we had to modify the inside baffles a bit but I think that was for the rocker adjusters. Regular 318 stamped valve covers might do it too, but we did not try them.