Can't wait to see your pics when finished, I've been reading this thread and it looks like you have some cool peices on your bike!
Answers;
1. yes there is a rubber mount for the lower tank bolt, it is rectangle shape with a circular peice that pushes through the hole from the bottom. then a steel washer on top of that. Hope you find it!
2. If the flush gas cap doesn't fit your tank it is probably a harley item??? The only way to make it fit would be to cut the nylon thread section off both your new cap and your stck cap, and plastic weld the stock thread onto the new cap you have.
