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I asked Salsa and here's what they had to say... So, the front is pretty straight forward – you can use a 180mm rotor on the Cuttroat fork by installing the rear standard, 35mm +20 adapter. I’m currently running this now with Shimano GRX. SRAM has the same adaptor available. The rear is more involved and is technically not compatible to set up a flat mount caliper with a 180mm rotor, as there is not OEM solution to go from a native 140mm frame to 180mm rotor. That being said, there are a couple aftermarket options – one would be to convert the brakes to a post mount system using a rear post to flat mount adaptor and SRAM post mount brakes that utilize DOT 5. SRAM does not officially market a post mount caliper that is compatible with drop bar controls, but I have successfully set up a gravel bike with a 2-piston LEVEL caliper. The only other option that I can see is to order one of a few adaptors that allow 180mm rotor spec with flat mount calipers. I have not personally tried any of these, but just ordered up this one to test fit - https://peak-torque.com/products/180mm-rear-disc-rotor-adapter-for-flat-mount-road-gravel-frames. This article poses one other option - https://bikepacking.com/gear/180mm-brake-rotors-on-flat-mount/. I DO NOT know if the ladder solution will work as it my interfere with the frame, especially under flex, but we also don’t have 3D of this adaptor, but the 3D below moves the Shimano adaptor into the position needed for a 180mm rotor.

This is a tough one to answer without having the bike on hand for some classic trial and error. If I had my Cutthroat still, I'd try some options to see what worked. It seems to me that any +20mm adapter should work, but, I think your best bet is to take it into a well-stocked bike service shop that might have some adapters on hand. I have used the Shimano adapters with other brand brakes, for the record. That bit shouldn't be an issue.