I've got a 55 bird, and an OT 67, both with A/C. Both have BeCool radiators with shrouds and electric fans. Both needed the alternators (generators were long gone) upgraded to keep the voltage up at idle. You get stopped, and everything comes on, it takes a surprising amount of power. Both units take a lot more power with the fan on high instead of low or medium. Both are integrated heat/cool systems.
The other thing I learned the hard way was the condenser fans. While they may be sized for the condenser, you need more fan up front to "help out" the radiator fan. The birds are noted for low speed overheating anyway. Guys in parades often pop the hood to get the hot air out. I think a 57 Merc would be way better in that respect. But as dmsfrr says, the whole systems needs to be working well, or the added loads will find the weak spot.
Under dash units are a bit tight in birds, but if I were doing another full size car, I'd keep the heater as is and put a good under dash unit in. They're making some pretty nice units now, some with "period correct" looks, some with remote ducts to the left of the column for the driver.