O'Reillys has the Anchor #2291 listed as fitting most 400, 430, and 455 applications '67-up, but not '67 or '69 Skylark. I believe '67 GS was a unique mount?
I've never seen an aftermarket '65'66 GS mount. Pretty sure it's been a dealer-only item since day one. A Chevy 2142 will work, but it's a 3 hole base instead of 4 hole, so the GS adapter will need a hole drilled and tapped.
I had 2 sets done by Steele a few years ago. Took less than 2 weeks and cost was around $85 a pair.