From searching further, I'm led to believe that part number 681-245 from Moss Motors may work:
MossMotors.com - Restoration Parts And Accessories For British Cars
They're polyurethane as well. At $10 for the set of 4 I just took the gamble and ordered them. I'll let you know if they work.
I'm sure I could find a universal bush kit that would work if I looked hard enough, but buying these kinds of things based off measurements never works out for me. I'd probably spend the cost of a new shock on shipping and restocking fees for parts that I had to return by the time I found the right ones, so hopefully these are good.
It would be enraging if I had to buy a new shock because I couldn't replace a $2 bush.