Standard o ring cross sectional area is listed in the chart I posted on the far left. My suggestion to you if you are not willing to correct the manifold/ head contact alignment, is to use the chart dimensions for standard o ring sizes and groove depth/ width. Then use the gaskets that you have been using to keep the manifold alignment correct. The o rings will sit on the gasket and fill any void that the gasket can’t.
If you go ahead with your plan, you will create more problems than you solve. The manifold floating on rubber will need constant maintenance/ attention. The manifold must be “clamped” to the head.