I swapped a 14-bolt into a 66 bonneville and I had the axle modified to accept trailing arms and coils. I'm not a fan of leaf springs. They are simple and unimaginative and the very flexible coupling they create is all that's holding it to the vehicle. In low-power applications or where ride isn't a priority, leaves are fine.
Keep in mind, retrofitting leaves isn't as easy as it seems. Getting the proper driveline angle isn't quite as simple under load as you think when engineering your mounts.
I agree with Bumpstick. That twin arm setup used for years is great.