Okay, I found the bag when I bought those well nuts and the note I put inside it. If you can't make out the label it says:
WELL NUT
FOR 5/16 HOLE
#8 - 32 UNC
ITEM NO. C23500
The note I put inside was for the screw size: Head diameter approximately 7/32. Length about 3/4. Threads 8 x 32.

Your other option could be to buy the reproductions by Tony's Parts, they're the 1970 style, and use Pal nuts. You obviously have to take the scoops off install his inserts.

Well Nuts.jpg