I'm a little baffled on how it could be a drop in replacement, for example on the 66740 pin 1 is /CS3, pin 3 /CS4, pin 10 /CS1 and pin 14 /CS2. With the 6574 they are VBB,VDD,N/C & N/C respectively. I don't have the Sol mobo out of the case currently to check the connections that are N/C, but I checked a VDM-1 board, and pin 10 for example is N/C, which would leave that /CS1 pin floating on the 66740, when it should be tied high. So ideally that should be done on the VDM/1 board and possibly, the Sol or an adapter made. Pin 14 on the VDM-1 also looks N/C on the pcb. Also I have not checked the implications of the other /CS pins vs the voltages applied to VBB & VDD. So as you say the 66740 may work just plugged in for the 6574, but there could still be issues..worth checking.