1/32 tarmac

i believe that you would want to go with a fairly fine grit, depending on how rough a surface you want for your tarmac. you may want to try a couple of different types to see what gives you the best result. I did a tarmac for a 1/48 scale p-51 and used a hard, but thin, piece of white cardboard. i then scribed in some lines to simulate the grooves, etc in the tarmac. then i dry brushed some darker colors into those grooves for the impression of wear and tear . it really came out looking very realistic. so you may want to try that too. :)

If you can go to a construction site and find some roofing felt (tar paper). They always have some they are throwing away and would not object to letting you have a few small pieces. It will make the perfect ramp. Attatch it to a base and go from there. Stain with thinned browns and black to show spills. It is the perfect item to simulate asphalt.

Bear in mind where your Mustang is based. Is it England in WW2? If so then the surface it was parked on would be concrete in most cases. That's no problem though. Stay with the wet'n'dry, cut it into squares around of four inches or so (for a smaller scale use smaller squares) Spray them in a light grey primer. Stick them to your base leaving a small gap all around for the expansion joints. Fill in the joints with thinned matt black paint (it won't matter too much if you go onto the 'concrete'.
Now 'weather' the concrete, spillages of oil, tyre marks (dark pencil lines) general dirt, pencil in some cracks,etc etc. Go have a look at some concrete road or parking area's. When you're happy, spray on a coat of matt varnish.
A good base can enhance your model.
Pete