You have allot of weight to get moving for a 4.3. Cost wise you may want to try to get the HP you have, work better. The 3.73 gears and the S10 torgue converter will help allot. Then have Bryan Herter do a PCM for you. A high output coil like MSD or Accel will help cover the Ignition. Basic like a cold air intake and some free flowing exhaust will maximise those areas.
Just adding a cam and some head work
as your first upgrades will be very inefficient unless the other areas are able to take advantage of it.

 

The cheapest way to get that kind of power is to spray it, barring that drop an LT1 in, then do intake, catback, and all the other little cheap upgrades. Trying to get 300 streetable NA HP out of a 4.3 would not be very cost effective.

 

I'm waiting for ghost to chime in. He had a 4.3 until he deep sixed it and put a Jasper LT1 in. Here's a link to a thread that might be helpful.
http://www.impalassforum.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=001057

Shoot ghost an email, which reminds me, his email hasn't been working the past few days, and he can tell point you in the right direction of getting your car where you want it to be. Good luck!

 

LT1 it! Thats the only realistic and practical way to get the power you want.

 

ask nory....