lighter?marginaly,less underseal etc and i think,slightly thinner gauge metal..........sounds trivial,but your going down the same route as i have,im looking for the last 1/2 bhp from my motor,and have spent time making the car as easy to propel as possible=more performance.
so you'll be looking for good used wheel bearings,not new,500 size wheels and tyres,running slightly higher pressure maybe,brakes(drums,discs have more running resistance) meticulously set up and maintained,steering geometry spot on(tracking etc)and loads of sound insulation..... to dump!!!.when you get your car,keep all the items you remove,then weigh them,you'll be suprised!
all small things to make the most of both our feeble power units!!!its tuning in another direction