I think you would do well to pick up a 90mm lens for 6x7 negatives. The Wollensak 90 I use was designed to cover 6x9 format, and no corner to corner light fall off on anything that size or smaller, really makes my 6x6's pop.
I have a Schneider Componon-S 4.0-80mm in like-new condition in original box available. It seems that it would fit your enlarger (I had the lens for a 4x5 Beseler & copy unit - 38mm mounting thread). Pics are available at www.vintage4-sale.com. Priced at $100 plus shipping.