I replaced the stock low-back buckets on my 68 Coronet with leather high-backs from a 71 Charger. The lumbar support is non-existant, but I love the style and I wanted to remain somewhat period correct. I agree modern seats would be more comfortable, but really, how often do we take our classics cross-country?
You'll have to figure out what's more important - comfort or trying to stay somewhat stock.
When I did my buckets, I put 1 inch foam on the bolsters, made a huge difference and also a 3/4 inch spacer under the front 2 bolts to lean the seat back.