The correct position where wings connect to the body, in my opinion, it's right over the shoulder place, as I've shown in the tutorial over the feline body.
The interior parts (muscle and skeleton) can be modeled as you think best, but from the outside the wings should pop up from there.
Also, if you really want to make an anatomically correct pegasus, it should also have the wishbone, coracoid and keel like birds have.
I don't have any idea of what's a SFM model is, sorry xD
Seagull's wing span is approximately the double of his body, so, minimum, the wings should be the double of the horse length.
But a horse shape it's more bulk than a seagull so the wings should probably be even more longer to support his weight (even if his weight would be lighter due to hollow bones)
Hope my answer is sufficient and understandable