what is the stock h.p. rating for this engine?

It all depends on which engine you have. could be 165-330HP. Check this out http://www.chevelles.com/years/71/71codes.htm

What barrel carb is it? My 71 came with 2 barrel carb so I assumed its 165hp net rating, I believe all the high ratings are gross since 71 is right around when they changed. Gross ratings are useless in my opinion. If yours came with a 4 barrel its probably around 200hp. Sounds pathetic but thats peak power. Modern engines have better peak power per displacement at high rpm. Still good power at lower rpm where the flow of components isn't as important. I slapped a edelbrock intake and carb on mine and it was good for a while, until it blew up from a combo of busted oil pressure light and leak. Never got to test the performance of it because at the time I had TH350 with 2.43 gears. I'd recomend getting a real oil pressure and water temp gauge if you don't already, the idiot lights are idiotic.

it comes with a 2bbl.

a 2 bbl 350 in 1971 was 245 horsepower

how much would it jump if i replaced the stock carb with a 2bbl holley,and headers?

Not a lot by switching 2-v carbs....maybe even lose some power....
Go to Craigslist or hit the monthly LB car swapmeet or the Pomona events....get a Edelbrock Performer intake with a Q-Jet or Holley 4-barrel from someone who has 'moved-up' ...probably $150-$200 for a nice set.....

OTOH, headers and a good exhaust help any engine......

Imho, the best bang for the buck here is starting w/ a good set of bowl-blended and dingle-berry, casting flash port cleaned-up double-hump heads to get the CR. back up to right at 10 to 1 using stock type, .020" thick steel shim headgaskets.

This way, you gain more power when you add a good intake system and then headers.

And to kick it up add the 268 VooDoo cam and go!!

pdq67

PS., and if you have the money, any one of many aftermarket heads are a step or MORE up from the old double-hump jobbers!

First thing I did to that engine was put on an Edelbrock 650cfm Carb and Edelbrock RPM Intake. You could also do a Holley 4 barrel if you prefer. A lot easier than installing heads :o and the intake doesn't cost a lot. The carb can be expensive but there are often some for sale used. I never did more than that. A few months later I had an oil leak and all the oil leaked out, the idiot oil presure light wire was cut and I blew Rod #7 through the pan.

The power increase you are probably after is up around 10 to 1 CR. so that you can run a bigger cam and still burn a good grade of pump gas, imho.

That's why I mentioned the head swap and double-humps should be relatively inexpensive if you find somebody that is taking a good set off and moving up to aftermarket aluminums.

pdq67