Kip,
Have a 58 with a 235 and 2 years ago converted from the 3.90 to a 3.38. A little more finesse is needed when starting from a dead stop with the stock transmission, and you will need that first gear, but it has made it a real pleasure to drive. I have had it at 70 with no issues, but surprisingly, I thought there would be a notable increase in gas mileage. As most of my driving is in town, I have noticed only about a 2MPG increase.
I really do recommend the change, but expect to use first gear regularly.
