Welcome Jim from Christchurch New Zealand.
I'm fortunate that i fit ok and dont need the lower seating but i think a combination of shock length and seat choice will help.
While the Corbin Gunslinger i have feels firm, they do say it takes around 500mls to 'mould' to the shape of your butt thereafter its way more 'fitting'and comfortable.
Mustang dont make a seat for the Roadster...yet. That's a bummer cos they are a great seat.
If you can get a factory seat thru ebay of similar, maybe an upholsterer could adapt it for you. It's been done b4 on this forum i'm pretty sure.
Meanwhile, enjoy the company within these pages...there are some good knowledgeable heads in here.