Bird, looks like we may luck out -- but just barely. Grandma has 3 wheel chairs -- a travel chair, a new one with removable arms, and an older one. The older one has a seat about 21" high; the other two are 19" high. The newer one, with the lower seat, is the one that she's most comfortable in. The top of the handles is at 36", and sitting as straight as she can. she rises another 11", or 47". The interior of the Windstar, at the lowest point of the roof, is 48". If she was still using the older chair, she wouldn't clear.
There is a minor problem, in that the door opening, at the lowest point, is 45". If she tilts her head a bit, or we tip the wheelchair back a bit after the wheels are inside and solidly on the floor (they go in before her head), she'll clear. She's still alert enough to cooperate. The strap retainers fasten to L brackets outside the wheelbase of the chair; it will sit on the floor without being raised.
I'm quite capable of installing the brackets, but I'm a little uncomfortable with the idea of deciding where they should go, so we're going with the professional installation for $600. On the other hand, there's no trick to the Tri-fold ramp, so we're ordering that from discountramps.com for less than $300.
I discovered one interesting fact. We have the dual sliding doors, left and right. The left (driver's) side door only opens to a clear width of a little less than 27". The right side door opens to a full clear width of a bit more than 30", so we can use a standard 30" wide portable ramp on that side. That's the curb side, so it would be the best choice in most cases, anyway.
Overall, we lucked out. I sat in the chair and measured the clearance I would need, and I would have to have 54" clearance with my head rubbing the roof, and there's only 48". Give me the same one inch we're going to give Grandma, and the roof would have to be raised 7" to accommodate me. That would have to increase to 9" if I used the taller chair, which would be more comfortable for me. I'm a not-exceptional smidgen under 6' tall.