Borla headers are by far the best aftermarket headers available for performance cars and trucks.
There are too many variables that determine what hp increase headers will give. Engine size, carbed,f.i.,stock,hi perforforman ce,car weight,the list goes on and on....
adding headers will allow engine to breath better and boost up horsepower but not by much, headers also make you vehicle louder with pipes.
Weight. Plastic is much lighter.
They have to weight 600kg. However, ballast weight is added to the cars to get over this weight, as the raw weight is somewhat lower then that (although few manufacturers actually will reveal exactly how much).
10hp
depends on the size of the headers and the size of the carb. low end.... 650 carb, small tube headers, you can expect about 275 horsepower
I would say your headers but theres lots of things to do to make it weight less.
A 350 SBC with aluminum heads and intake would weigh approximately 450-500 lbs. Adding carburetor, headers, alternator, water pump, and other accessories would increase the weight to around 500-550 lbs.
You will probably need to use block hugger headers. The fit will be VERY tight because there isn't much room for headers on a small block. If you remove the inner fenders you will have much more room but there will also be a lot of draw backs to this. Check with Hooker or another high quality brand name. Don't waste your time with flotech. They won't have what you're looking for. Here's a suggestion. I know that the same headers will fit on a 72-76 Impala and a 77-90 Impala. It is possible that a set of headers for a 72-76 big block car will also fit for the 77-90 cars.
They weigh about 37 pounds.
If you think about it yes they would cause headers flow better but when I say they will I'm not sure how much louder they will make your vehicle.