I'd want to get it as close to 21 columns as possible (being optimistic thinking about a 1590B just in case :o), and looking at the transistor section I reckon that could be done in two cascading rows like my Muff type layouts and so maybe 15 to 17 rows. Then two opamps I could potentially get in 12 to 14 extra rows, so I reckon it would end up between 27 and 31 rows. A few people have got the Snow White Auto Wah into a 1590B and so it may just squeeze in, but otherwise I reckon it will almost certainly go in a 125B unless those transistors are awkward to place in a way that makes the best use of the space.
I'll have a closer look soon.