Bob,
I am not sure about the 40 gallon tank from a new suburban. I had an old 30 gallon tank out of a 72. I was going to put it in until I saw the new 27 gallon stainless steel tank kit offered by Early Classic Enterprises (559) 291-1611,
www.earlyclassic.com
This is a complete kit for the sub, and its a lot better than the 18 gallon poly blazer tank!
Rich