Your g/f, whom I assume is hot, or you wouldn't be putting up with this ****, is having one of several reactions, (1) he's not spending enough time with me, or (2) he's wasting his time on something pointless, (3) working on a car is degrading, or a combo of the above.
For (1), the best way to solve this is to take her out for some together time. Then, when that together time has gone on for a while, and you've presumably both had a good time, tell her you want to talk about the Mustang, and how it's very important to you, and that while it is important to you, you want to come to an agreement on reasonable times to work on your car. Set some sort of schedule with her, and even if it isn't any different from when you work on your car now, I guarantee her mood will improve because she'll feel included, that her opinion matters to you, and that you were willing to compromise on the Mustang for her sake. Which are all pretty big things for girls.
For (2), you have to explain the Mustang hobby in girl language. Whoever hit upon the "shoes" thing brought up a very important point: girls need things explained to them in terms they understand.
Car guys spend time leafing through car mags for parts ideas; girls spend time window shopping for shoes.
Car guys can spend endless amounts of time working on their car; girls can spend endless amounts of time trying every single possible combination of shoes/accessories/hairstyles/clothing before deciding on an outfit to go out in.
Are you asking her to give up her shoe collection? Of course not. Is it hard to understand why she needs 50+ pairs of shoes, most of which never see light outside of the closet? Of course... but you don't ****** about it, you realize her shoes are important to her, and it's something that you wouldn't try to change about her. If she has a hobby, you can use that to strengthen your argument, but the shoes argument worked well with my g/f-turned-wife. She still doesn't understand fully the whole working on the car thing, but she doesn't try to change it about me either.
As for (3), believe it or not, a lot of girls equate working on the car => trashy, because of several things: all of the ******* ricers they see who work on their cars, Jeff Foxworthy (that 12 Redneck Days of Christmas song does NOT help), or they believe it's beneath somebody to work on their car when they can afford to pay somebody else to do it.
For this one, you'll have to explain, in detail, why those stereotypes exist and why they don't apply to you. You'll also have to assure her that you're not becoming a ricer wanker or a redneck.
My then-g/f was mostly (2), with a little (3). Now everyone's (mostly) happy, and all it took was good communication.