There was no single-carb adapter. The purpose of the cross-ram was to increase [1,200 cfm] fuel-flow over the production 780 cfm carb. There is no reason to believe Chevrolet Engineering even considered it; the SB hemi-head was a clear failure but is listed in the SCCA/FIA homologation documents. A single-carb adapter is not.
That is correct. For 1970, a single 4V carb was 'required' for SCCA Trans Am racing. So, as I believe Chevrolet went back to a single four intake. I'm sure Jon (and others) will chime in to educate us what exact intake was used for 1970. As a side note, the Brand X models had the options of either a 4V, or an 8V tops for competition use.