The difference would have to be the diameter of the bolt holes on the convertor. 10" would be FRA-302 or an 11" would be a FRA-413 and either one is readily available normally in 24 hours at Oreillys.
The main difference is the diameter of the center hole.1967 down is 1.960-1.965, was measuring a used flexplate.
1968 up the center hole is 2.160, approx .200 difference. The crankshaft bolt holes are in the same location, for both the early and late flexplates.
The later flexplate is available from several sources, as new. Since this thread started, I have not been able to find a correct listing for the early flexplate , new. On Monday I can make a few phone calls to some contacts I have, and see what I can find out about the incorrect listings, and see if any one has the early flexplate available.
I don't ever recall seeing an 11 inch slant six 904 converter. Not saying there is no such thing, just I can't recall seeing one.