You are better off in my opinion to build the '63 valiant. On the '70 dart , the biggest gain or weight loss I could say would be the hood (as far as front end fiberglass parts go). The stock fenders really are not that heavy at all. And even worse, often times the fiberglass fenders have a terrible fit. You'd be miles ahead to start with your valiant, as it's a lighter car to begin with. If you went the fiberglass route, about the only thing that would get lighter is your wallet- or rather for the money spent you would not see a noticeable difference in weight loss.
I used to run a '71 duster and it was just to nose heavy- Getting it light seemed like an uphill battle. After I gutted it to the max, I figure it still weighed about 2800. I figure a '70 dart would be pretty similar -my guess.
You could get the '63 to around 2400ish without to much hassle (including the roll cage) etc.
Of course Im not telling you not build the dart- Im just trying to save you a few dollars in the long run.
The valiant's from about '63-66 are about as light are you are gonna get.
The really low optioned valiant 100's make a good drag platform.
I will be doing a '66 barracuda that's a little heavier than a valiant, but after Im done with it- No longer an issue. The 64-66 b'cuda's back glass does weigh pretty close to '60 pounds
I had even thought at one time to do something weird, like a '60 fiat with the suicide doors. But by the time you add the cage and the safety equipment and make the car sturdy enough to handle the additional hp and torque, you are right on par with the weight of a basic a body. Then of course the slant six bolts right in. No fabrication headaches, even though I do like to fabricate
Good luck with your choice
