While you will replace many items before racing any car, I would give the chassis a careful lookover as theres no point in building on a rust bucket or a twisted unibody. And the transmission might be trash. The engine too might need "refreshing". So, after you do all the replacing, (the entire brake system, all the bearings, and anything else thats worn, like bushings- uh oh!) and you get the right rear end, the safety stuff...cage, straps, net, etc. you've got yourself in for a few thou min, if you don't do much performance stuff. And it will take awhile.
So....why not buy a built 7? Plenty to choose from, and many are cheap. You can run it in IT7, and ITA, but a good ITA field will give you fits. The best of the 7s at the ARRC were about 5 seconds of the pace of the lead ITA cars. Do your homework, as there are a number of diferent ways to do the susp, and you may find that you like some setups better than others.
That said, if you have the space to house the car, it will make a great spares car. Or get your sawzall out and strip it.
Good luck!