I have a DD 65 dart with 74 A discs/10"drums with an alum 1&1/8" alum M/C with the original drum splitter/no prop valve. On paper it is mismatched but it stops perfect & no discernable wear on the pads. In your case I'd do the swap & leave the M/C as is & as noted keep an eye on the fluid but I wouldn't sweat it. When you get the discs on there have a helper press the pedal & see exactly how much the level drops