Thanks for posting your pics, Kathleen, especially the before/after w/the car. This is an inspired piece of work. You did a great job with the car swap.
First impressions: When I opened the image the first time the thought "What's wrong with this pic?" never crossed my mind. The car doesn't look out of place or angled wrong to me.
Food for thought...
Color: This was a typical color for this era of cars... kind of a pea green. Painting it yellow would make it look very out of place (to me, anyway).
Front wheel: Looks a little flat. Maybe isolate a copy of the back wheel on a separate layer, position it over the front wheel and layer mask a little air into the bottom.
RE: Maybe turning the wheels a bit.
That has possibilities. The most natural position for turned wheels would normally be towards the curb (probably not the direction you envisioned), but since there's no curb, why do that?
Rotating the wheels out would add the slight complication of finding (or painting in) some tread. Challenging, but not impossible. It would be a + if you can pull this off w/o making it look obviously fake.
RE: Car paint
Towards the bottom of the body, along the door bottom and just above the left-rear fender... see how the paint is darker? I don't think it would hurt to sample the lighter color and on a separate layer airbrush over those dark patches. That dark area of paint looks a little out of place to me.
Other stuff:
Perhaps find some other images from about the same era as the old car and "plant" one or two to give it a little more old-time flare. I have no idea "what" these 1-2 add-ons might be (an old streetlight or billboard-type bench perhaps?), but if this was a pic of a cowboy for example, putting a horse w/saddle nearby would give it flavor.
Hope this is this the type of feedback you're looking for. When you get the final version complete, sure would like to see it.
Danny