Yes, the 66 has a horizontal bobbin. I know it is said that vertical bobbins are better for FMQ. I know that the threading is different from regular sewing than it is for embroidery, darning and probably FMQ on the 66 and 99. There are a couple of threads regarding the threading on the 66/99 machines. The one that has a diagram is
2 Vintage Singer Machines, 2 problems. but it shows the the normal sewing threading the illustration for the embroidery darning is generally toward the end of the manual.
Another that mentions the different threading, (no diagram) but is about FMQ on a 66 can be found at
FMQ 1930 Singer 66 problem
While there is a feed dog cover made for the 66 and 99 I would imagine that
https://shop.sew-classic.com/Feed-Co...nes-P60402.htm would work also.
Another thought would to change the the amount of pressure on the presser bar. I know Cari has a broken one on a 15 that she can FMQ with.
Janey - Neat people never make the exciting discoveries I do.