Have you looked at the following gas tanks as an option? I have seen nice installs and they would require no removal of crossmember (on my 46).
Give me a email and I will send you a picture of an install. Hovarth has good pictures of his but it is not in a deco.
High quality Ni-terne plated steel gas tank with 3-year warranty
Cost: 146.00
Applications:
Blazer (Full Size): 1973-81 31 gallon
Jimmy (Full Size): 1973-81 31 gallon
Suburban: 1973-81 31 gallon
Features: Small filler pipe, 1-1/4" ID, Depth of bottom half 5-1/2"
Capacity: 25, 31 gallons (depth diffences)
Dimensions: 28-3/4" x 28-1/8" x 12-3/4"
Free with Tank: Rubber O-ring and lock ring for sending unit
Packaging: Boxed
Part Number: GM-15A and GM-14A
cost: 146.00
http://www.quantaproducts.com/products.asp?dept=30Cavebull