Several companies specialize in discrete hang on systems that provide plenty of cold air without detracting from the natural beauty of the interior and engine compartment. Vintage Air appeared on an episode of Jay Leno's garage and provides ready to install AC systems on vehicles going back to the late 20s.
They also offer sure fit systems that are designed to replace defective air-conditioning on popular muscle cars from the 60s and 70s.
Kits are manufactured for vehicles that came with air-conditioning and for those that had AC deleted. Classic car collectors often say, there is nothing money can't fix.
Hope my experience with has become. PS - If you are topping up just put ml in to offer with and then test the dip stick to see where you are at. I put in ml and was way over the Max Warm Mark on the best and my reservoir was bone dry.