the big thing is oil return and water flow.. most often the coils are designed so that the water runs vertical down netween the fins and drips in the pan... if water is allowed to Pool on the fins the fan will end up slinging it off and past the catch for the tray.. equals drip on the floor.. (GM had this issue in the 60s and 70s and put a non serviceable plastic screen on the back of the evaporator to catch the water (and lots of dirt).... Nice one GM! grrr) you also dont want to trap oil so you need to see how your coils are designed so that all the 'U's in the ends dont become oil traps.. all these units are different ..
one thing to think about is the newer units are being installed in the bulkheads.. that empty useless space above the rear windows and front windshield.. it might be worth looking into bulkhead evaporators.. not cheap but they are bad ass.
these guys are high priced.. you can probably find them cheaper elsewhere..
EVAPORATOR BULKHEAD BH20---RAC