I don't run a dirt track car, but have some ideas that may work. first the best chevy iron heads are the 441's and 492's casting but are a little had to find. Below is an article by the late John Linderfelter on those heads. Second, run the 4 barrel manifold with an adapter to a 500CFM 2 barrel holly carb, they work well. I'm not sure how much cam you can run and still pull 15" of vacuum, but i'm sure you'll have to have around 110/112 lobe ctr's. If you can find a stock steel crank use it, otherwise buy the cast steel after market crank, and what ever you do be sure to replace the stock rod bolts with good ARP bolts, use the 2 valve relief pistons and mill as much off the heads as you can to get the compression up. The 461 and the 492 heads are 64 CC's, put 2.020 intakes with 3 angle valve job. You could easily get 400 HP from this combo.Also use the 1.6 long slot rockers.
JMO
Zip.
http://books.google.com/books?id=GUC...0heads&f=false
http://cgi.ebay.com/ebaymotors/Steel...Q5fAccessories
http://cgi.ebay.com/ebaymotors/3-Sol...Q5fAccessories