I did take my car to an A/C shop after I replaced the evaporator AND put everything back together. If you notice, the evaporator inlet and oulet is located in the engine compartment by the firewall, on the drivers side. So, my logic was, if you have a leak, it would be outside the dash or, in other words, "reachable". If your evaporator is new, I would not worry about leaks or, you can pressure test it BEFORE you install it. Do check ALL the vacuum lines under the dash to include the "pods" and replace them while you are there.
Did you remove the steering wheel?
Good luck!

__________________
J.H.
'86 300E