While re-reading Miller's book on the B-58, I found the following:
"1957-ln March, a Model Improved B-58 was proposed as a "split mission" aircraft. In this configuration, the fuselage was extended 8'; one crew station was eliminated and the two remaining crew members were placed side-by-side; the design gross weight was increased to 185,400 lbs. maximum; fuel load was increased to 116,800 lbs; wing area was increased to 1,581 sq.ft, and the wingtips were made foldable. Additionally, the aircraft was equipped with a more powerful ECM package and General Electric X-207's (advanced J79's) for propulsion. One interesting feature of the growth version was the proposed use of jettisonable outboard nacelles. This was designedto increase the B-58MI range performance. The B-58MI was eventually referred to as the B-58B (see 1958)."
and
"1958-During the spring, the Model Improved B-58 as proposed by Convair was revised and presented as the B-58B. In this configuration, the
aircraft had nose mounted canards and more powerful J79-GE-19 engines."
In November of 2010, OBB posted a picture on his blog of a B-58 variant that is probably closest to the 'B' of any illustration so far, and appears to closely resemble the first quote above.