I have heard such good reviews of the Monroe Load Leveling shocks from the '59 guys, that I bought a set for both the '58 and '59. I think the shocks on the '58 were last changed around 1972. They were completely shot.
I had put standard gas charged shocks on the rear of my '59 about 5 years ago and was not impressed with the ride.
This time, I went with the load leveling, coil over shocks on both cars and so far have been very impressed with the handling.
For the '58, I used Monroe 58567. They were $61 from RockAuto or you can buy them from FLAPS for about $75.
For the '59, I used Monroe 58510. No modification necessary.
For the '58, I did have to slightly modify the rubber grommet at the upper mounting point, as the new grommet was about 1/8"-3/16" too large to fit inside the mounting bracket. I also reused the steel collar from the old shock since the new one was the same amount too large. The modification was minor and took only a few minutes.
I put standard gas shocks on the front.
Ride quality is seriously improved and handling is incredible!
I highly recommend these shocks. Don't wait until it's time to change them, upgrade now! You'll be glad you did.