Over in Lincoln is the headquarters for Masport . They build industrial vacuum pumps and vac truck components. You might learn something surfing their site or even call them for some tech help. Or they might refer you to a truck shop/dealer/distributor that could help you. Safety is the most important thing. Does the old anhydrous tank have a data plate that gives the pressure rating? It's probably more than adequate - if its not corroded and thin on the inside, where you can't see. Your 20" Hg rating is approx 10 psi of external pressure trying to implode your tank. Apply that pressure to maybe 18000 square inch inside area and that's 180,000 pounds total force. Be careful!

Professional Novice