This can work, but be careful, it also does not address the root cause of the problem...gunk, debris and clogging of the water output of the heaterbox (ie, your new evap core will be leaking again in a couple of years). You also are unlikely to get good sealing around the evap core by doing it this way.
I'd still stick to pulling the whole box out, giving it a complete cleaning, getting all the gunk out, replacing the heater core while it there...and knowing the system will work well for the life the Jeep.
The evap sits perpendicular to the firewall. If you run the Jeep and have the air running...fell under the box, one long section will feel cold (this is where the evap core is), and one will feel hot (this is where the heater core is...sits more parrellel to the fire wall).