It should be a 50 ohm antenna. RG-8 is fine, NOT radio shack junk.
If not RG-8X (or M) is fine. If you use the 8X or M, you will need a
reducer.
THe one for 59 U is the one you will need, NOT a reducer for 58U. RG-8X
is the physical size of RG-59U (cable stuff)

For CB use, the RS RG-8m is just fine. You need RG-175 reducers for RG-58,
and RG-176 reducers for RG-8m. I'm using Belden "Truck King" RG-8m that I
picked up at a CB shop some years ago. I'm also using a bunch of RS RG-8m
for short runs on 2 meters. (20 feet maximum) For HF, RG-8m is fine for
runs of up to 50 feet. On VHF you want something better if you run more
then 15 or 20 feet. Couplers get touchy at VHF. Imagine UHF!!!