I think the 15-91 is about 7.5" throat. There is an easy mechanism underneath to drop the feed dogs. Very strong machine, will sew through almost anything! (leather, etc)
Also, easy to find. ;)
I know where there is one for sale in york county, pa. $70, the "twin" to my machine. Pm me if interested.
If you dont mind industrial singers, the 30-15 and the 241 have 10-11" throats. The motor hangs underneath, though, so they would need modification to go on a frame.