Dont waist your time with the big block, trust me they are not worth it. He is right you will spend so much money building a big block, when you could build that 350 to run 12's for half the price. Twelve is not just a number that you can throw out there, you are going to have to do alought of work man, and i mean dropping your suspension out in the back and building your own just to get the tires to fit up under there. I would build it with aluminum heads there alought lighter then iron and they disipate heat better. You will want to get a stroker kit for that motor witch is not cheap but once you install the new kit it will push your displacment past 390 somewhere around a big blocks displacment witch is ****ing awesome, because then you will have this light little engine putting out just as much as a big block would. But before you get the stoker kit fix your upperend new heads, springs, valves, retainers, new roller rockers with a roller cam, with a good lope to it. New ignition system i would go with msd, some good headers probably heddman's, Get a good exhaust system. If your talking 12's for exhaust you might be able to get by with some good flows, or dynomax, but i would go with borla if you have the money. You are going to need to get a locker in the back, i would go with a detroit locker they are really good, you can also go with an air locker but there alought more expensive. You are going to have to throw some better gears in the back i wouldnt go with 411's though that is to lto high you will top out before you hit the quarter mile mark. My brother built a roadrunner with 4'10s and he topped out at about 70 but that was becuase he didnt have overdrive. After all of that you still have work to do you have to break the transmission off the block and putt a stage 3 shift kit inside of it, witch will be really hard on it around town so if you want it to still be drivable without damaging your transmissionn then go with a stage 2. Your pretty much there man you are also going to need about a 750 cfm carb to go along with that and a new intake manifold along with a new distibutor preferably msd or mallory. Now you need to get new push rods and have your block bored out. And after all of that you are going to want to get a supercharger probabaly from packston for about 2000 bucks , and you should be good to go lol, you might even be running faster then that with all of those parts.
PS once you get all of tha you are going to want to have a proffesional tuning shop to tune it for you.