[LS short box]

I used ni-copp? tubing from front to rear on my 68 C10. It flares like butter. I bought it in bulk from Oreilly's. I do have a good flaring tool and have used it with good success on stainless but as said above I had to do some rework because I had some leaks.
Also if you do decide on stainless there are little concave washers made out of copper that can be put between the flare and the fitting to help seal the line to the fitting. Sorry I don't have a link.
The thing I've found on flaring lines is the line prep before flaring. After cutting deburr the inside of the line. I use a small center drill. The a slight chamfer on the outside the line. Some lube on the flaring tool as you make the flare helps.