Howdy:
A stock two piece retainer is not machined for a damper lip.
I'd suggest that you recheck your numbers. 200 open sounds ok, but 120 closed is too high for any commonly used valve spring we see on a 200. Check the springs without the dampers. Also, the 1.68" and 1.23" heights would be the check heights for a relatively stock cams lift of .360" - 380-". If your cam has .450" you would need to check spring pressure at lifts specific to that cam. And be sure to check for clearance between the bottom of the retainer and the top of the valve stem seal.
The stock locks are quite stout. I've not heard of locks giving up, but there's always a first. If the open pressure of 200 is correct, then I wouldn't be too worried. That's not too over the top. The single groove Ford style keepers are used on some pretty fierce engines. If there's a weak issue (and it's a small one) it's more likely related to the smaller diameter valve stems of the six cylinder valves. The locks are correspondingly smaller too. Less material= less strength and durability.
Adios, David