You might try cleaning the wheel that feeds the wire. Use a small wire brush. They tend to get clogged with wire filings sometimes and want get a good grip on the wire stock. If that doesn't work you might need to replace the wheel. Also take a close look at the contact tip. If the hole is getting closed up with slag residue it will cause feed problems. I replaced the tip on mine a few weeks ago and it made a big difference.
Here's a pic of a friends '54 that I welded in a patch where the filler neck was.
filler neck patch I used a Campbell Hausfeld flux core wire welder to do the job. Just requires a bit of grinding to smooth it all up.
