The ME60 you mentioned is for a 2″ driver; the DE250 is a 1″ driver so you would need some kind of adapter to make that work.
Within the B& C line, the ME20 ( square) and ME45 ( rectangular) would match up. The ME45 can load down to a lower frequency, so that might be a better bet with the 8″ driver and it would also reduce the center to center spacing which is also a benefit. If you would go the rectangular route, stick with the H290 since the high pass part of the crossover could probably be used as is.
A different driver/ horn combination will be a start from scratch proposition for the crossover.
The impedance curves from 900Hz to 4KHz ( crossover zone) look pretty similar in value for the 4 ohm 6″ and 4 ohm 8″ drivers, so the low pass part of the crossover may also work pretty well as is.
Should be a fun project for you.