The information I have says that 3/8 T-slots will be .438 - .448 wide at the top and .719 - .781 wide at the bottom so those should be the ones you want. I think the 7/8 figure is probably how long they are.
Enco and others often have specials on T-bolt sets. Probably imports, but mine have worked for everything I've needed.
"I am often asked how radio works. Well, you see, wire telegraphy is like a very long cat. You yank his tail in New York and he meows in Los Angeles. Do you understand this? Now, radio is exactly the same, except that there is no cat." : Albert Einstein