As far as I know, Cat 1 refers to the pin sizes and load capacity, not necessarily the spacing. If you're unsure, measure the distances side to side where the lift arms go into the blade you're thinking of getting, and the height from the lift arms to the top link hole as well. Then check how far apart you can spread the lift arms on your tractor without rubbing the rear wheels. Same with top link height.
Typically, if your tractor is 54 inches wide, for example, most implements you choose will be at least that wide, probably 60 inches.
The rear blade is an exception to that rule, since if you want to plow with it angled, and you will, it needs to be wider to account for the width lost when you angle the blade.
My L3400 takes a 60 inch rotary cutter, but if I were buying a rear blade I'd shop for a 72 inch because of the reasons I mentioned.
You may find it's a bit too big if you want to move a lot of heavy soil, just take a smaller bite (depth).
HTH,
Chilly