It will fit, but a 91's radiator has an extra hole in the driver's side tank for a switch that you don't need (I think; I'm not sure when Chrysler got rid of that switch), so you'll just need to plug that hole--nothing a trip to the hardware store for a plug won't fix.
I know you want to save money, but a brand new OEM replacement radiator can be purchased for less than $100. I'd never buy a used radiator. You have no way of knowing if it has any clogs/restrictions or not...or if it has been put through any kind of crazy abuse...or if stop leak once flowed through it (you don't want that crap in your system). The tiny bit of savings are not worth the risks in my opinion.