Building up a Dart

The biggest difference on the gas mileage was not so much the weight but the undersized carb and bigger exaust. Darts are very well balanced in terms of weight that it does not make much difference. (Ask anyone who swapped in an aluminum for a cast iron slant) If you are worried about the weight then I would keep what you have, get fiberglass bumpers and that is all you can pretty much do for the body. If cost is important to you then I would build a 400 stroker (to make a 451 and you can pretty much build it all yourself w/o worrying too much on the casting.) On the flipside- If you are willing to invest- a small block with aluminum intake and performance heads will keep the weight down and win a few races. Or you can go to the Darkside- find some stuff on slantsix.com and try to push your slant to the limits. (Not cost effective) Too many thoughts. I'm probably no help but I'll vote the big block. Just my 2 cents.