It looks like the nibbler has a shaft that protrudes out the bottom and oscillates I don't think there will be enough headroom between the tank and floor. You might want to look at getting an air/electric shear You can find them at tractor supply or Harbor freight pretty cheap. The Harbor Freight Central Pneumatic shears are on close out for $13.00 if you have a compressor, probably junk but should last long enough for what your doing.
https://www.harborfreight.com/14-gau...aign=shortener
https://www.harborfreight.com/pistol...&cid=go_social