Headers hitting idler arm

I'd do 3 things .
Loosen all the block/mount bolts, and lift the engine to gain clearance at header and relay rod. Retighten bolts lifted.
If not enuff clearance gain, order 70s van insulators, there thicker.
You can file the idler arm holes in K, lowers idler, to gain clearances nec. then tack flat washers over elongated holes to stabilize.