The whole KC-X procurement was mired in controversy because there was no good replacement for the KC-135 before the program began. The KC-135R presented a set of rather oddly matched specifications, with OEW of just 56 tons compared to fuel load of 91 tons (MTOW 146 tons) and a very small ramp footprint. Modern narrowbodies top out at about 100 tons, so there is clearly not room there for something to match the Air Force's requirement for 90 tons of fuel.
The smallest widebodies in current production were the Boeing 767 and Airbus A330. The Boeing 767 has a minimum OEW of 80 tons and a maximum MTOW of 204 tons and the Airbus A330 has a minimum OEW of 109 tons and a maximum MTOW of 242 tons. The KC-X competition came down to the Boeing KC-46 (OEW 82 tons as in 767-200ER, MTOW 188 tons as in 767-300ER) and the Airbus KC-46 (OEW 125 tons, MTOW 233 tons). The 767 is both smaller and older than A330, so it makes sense that it would be cheaper to buy and operate.
I am thinking that the Air Force's target weight and size for the KC-X might have made it a viable target for an MoM size airliner instead of the smaller widebodies, if any such aircraft had remained in production into the 2010s.
In terms of weight and physical size, the Boeing 757 is the closest match with an OEW of 52 tons (757-200F), but MTOW of only 116 tons. Fuel load is about 35 tons, but that is with standard tanks. I don't even know if it would be possible to increase the 757's MTOW by 30 tons. Honestly, I think the OTL 757 design is just slightly too light for the MoM size slot, and I would look at something closer in size and weight to the Douglas DC-8-62/63, about 65 tons OEW and about 150 to 160 tons MTOW, which would coincidentally have the weight reserve for the Air Force's 90 tons fuel requirement.
For Airbus, the closest option would have been the A310, which was very comparable in size to the 767-200, with OEW at 79 tons and MTOW at about 160 tons. The A310 MRTT tanker conversions used by Germany and Canada can carry up to 78 tons of fuel, which is better than the 72 tons carried by the KC-767, which was the predecessor to the KC-46. Considering the historic sales performance of the 767-200, I don't think a 767-7 MAX or an A310neo in the weight range would perform particularly well. However, I think a new-build MRTT derived from the A300 or A310 would have been better matched against the KC-46.