The question as, at what point is breathing room worth more than milimeters of radiator? I'd actually love an answer to that, but Necere's recommendation of 25mm is very reasonable. Of course, it's not that simple as there are other variables, e.g. radiator fin density, fan RPM, fan static pressure, flow rate, and ambient temperature.
And also, any radiator thicker than 45mm you will need a push-pull configuration for, unless you want to run your fans at >2500 RPM which isn't really tolerable, and of course push-pull is not possible in the M1. Unless the rad is very low fin-density, but even then 45mm is too much in my opinion, and I think 30mm rad of high fin density + 25mm fan + 30mm breathing space will cool better than 45mm rad of any fin density + 25mm fan + 15mm breathing space. It's a hard optimization problem, but think the sweet spot is a 30-35mm rad of around 20 FPI with good static pressure 25mm fans.
Regardless of any of that, a 60mm rad is an awful idea. Will it fit and will it cool? Yes. Anything close to optimally? Not even close. Your fans will be sitting against a wall for half the length of the radiator, for the other half they'll be sitting right up against the CPU, RAM and motherboard.
Even if you switched to a bigger case, a 60mm radiator is overkill. I honestly think in any case (no pun intended), you shouldn't go thicker than 45, then you're just hitting serious diminishing returns. A 360mm 30mm thick 20 FPI quality rad can pretty much handle any OC on the CPU and single GPU of a modern system I think.
Thanks for the well thought out reply and putting things in perspective. I was also considering a radiator at the bottom of the case. The nemesis 240 gts with slim fans, but based on what you said about fans being pushed up against things, that would sound like a bad idea too. I guess just 1 nemesis gts on the side panel with 25mm fans it is.