Any tricks for finding a leak in fuel line ?

I had the same problem. Only spot I ever found where my fuel line might have been getting a little air is where the rubber line attaches to the metal line at frame rail. OEM line had a crimped clamp on it. It was a little loose. Repairing that made no difference.
Later models have the fuel filter positioned vertically down near the fuel pump and a thicker base gasket under the carb. Filter fills and fuel in it stays a little cooler down there. I made those changes to my 67 so I could run todays fuel. Once the non ethanol "real gas" became available I run it in everything here that has a carburetor. I haven't had to replace a line or primer bulb on the weed wacker since. Real gas costs more but well worth it to end all the fuel delivery issues I had for years.