73'-up seat foam

I know that no one sells seat foam for these seats, and my backs are workable, plus I am going to add an adjustable lumbar support due to a wonderful thing called ankylosing spondylitis. But....

Anyone know if 70-72 seat bottoms are the same, foam-wise? Mine is workable, and I am sure I could add some higher density foam to some parts to make it more comfortable, but I would like to know if there is an alternative just in case they are worse than I thought when I tear them down.