Gas tank spewing gas!

[bigdon-70blazer]

I got the Blazer fired up tonite! Ran for a while and now we have to do the details but it ran for a while and we shut it off. Not traveling under own but soon. Did do something curious though. Poured gas into the tank and after a short bit, tank rumbled and spewed gas back out the filler. All over me but that is not important. I used another 1969-70 era gas tank after I determined original had leak. Cleaned the one I put in well and replumbed the entire works with new rubber gas lines where needed. Used the old sender unit as it was in pristine condition, only 56K miles on rig. The gas in being pumped from the tank to carb so that works fine. Haven't got enough gas in it to determine if the gauge works yet. Any ideas? I am going to trace down the vent lines tomorrow but thought I would ask. Also, need throttle solution for the new holley 600 carb. I removed the old quadrajet so you know what my linkages looked like originally, they don't work with this set up. Thanks. Starrted after almost 4 years!