When the DACTs arrive, you can connect them using the series resistors as used in the original FP-III - it is set up so that there can be different series resistors for each input. In the manual dated 2005, at the bottom of page 24 there is a photograph showing those resistors fairly clearly.
Keeping the 50K control stock will mean you'll want something like 100K series resistance to get back to the original default -10dB. Alternatively you can shunt each 50K with 22K or thereabout to make it approximately 15K, and retain the stock 33K resistors.
Note that the original FP-III had an unusual volume control connection, intended to make the switch less noisy when the contacts were erratic. With the 50K you'll most likely prefer to use a standard potentiometer setup, except for the series resistors.