Well, apparently each MC88920 on every Quadra 605 motherboard I've tried will take 20 MHz in on the SYNC line and produce something close to 40 MHz. Granted, the speeds from each differ a little - Clockometer shows 38.something on a couple, the "cpu: delay factor 1593" line is a little different on each (1593 is from a 50 MHz m68040; a true 40 MHz is around 1280, and some of the 605s are in the low 1200s and high 1100s which would put the actual clock between 36 and 39 MHz).
The reason I brought this up is that most people are not capable of desoldering and resoldering surface mount chips, so if you can get most of the speed with a lot less risk, it's worth sharing.
On the other hand, I just bought a lot of seven MC88916DW70 on eBay, so if anyone would like one or two, email me - I'll sell them at cost ($4.50 USD plus shipping).