Before I ask about specific gourd varieties, I would like to ask three questions about gourds in general:
1:Will gourds grow in clay soil?
2:What is the minimum amount of sunlight required?
3:What are the pros and cons of growing Gourds on a trellis vs on the ground?

4:Now, about the specific Gourd Varieties:
I'm thinking of getting two different gourd varieties to use, one for Purple Martins and one for smaller birds (Carolina Chickadee, Tufted Titmouse, Carolina Wren).

I know the traditional variety of gourd used for Martin houses is a birdhouse gourd, but given the latest recommendations by the Purple Martin Conseration society (Gourd width 12-14 or more inches wide with walls 1/4-3/8 inches or more thick) I'm thinking maybe it would be easier to reach these dimensions with a larger gourd variety.
Possible alternative varieties:
African Wine Kettle Gourd?
Basketball Gourd?
Bushel Basket Gourd? (Smaller than African Bushel, most likely still too big?)
Corsican flat gourd? (Might be too low?)
Zucca Gourd? (probably too tall?)
Others?

4A:what are the approximate average wall thicknesses (in inches or millimeters) of the above gourds, and which ones have the most durable shell?

For the smaller houses, I was thinking a gourd at least 6" wide,(preferably closer to 8"-9") as thick-walled as possible, and possibly with a thick curved neck that's 4-10" long and no less than 1&1/2 inches thick on the inside of the thinnest portion of the neck (preferably closer to 2"). I think if I put the entrance hole on the underside of the end of the neck, it would make a great built-in (Grown-in?) predator guard for everything exept snakes.
Possible candidates:
Goose/Duck Gourd?
Water Jug Gourd?
Apache Dipper Gourd?
Lump in the Neck Bottle Gourd?
Indonesian Bottle Gourd?

4B: Which are the most durable of the above gourds , and how thick are thier necks on average?