well, i'm not sure on costs, but i'm sure if you look around you can find the GN engine. I believe several models had similar engines, look up the GMC Typhoon, Cyclone, and a special model of Firebird (for some reason I think they called it Indy Firebird). Yes these engines are rare, but they should be availible. Me personally, if I had that year Monte I would either invest in building up a 327 with a roots blower on top and a large carb (holley or edelbrock - I have heard the goods and bads about both of them). Why 327?- I dunno, I just like them! Follow up with something like 3.55 gears to get what gas mileage you can from it. I'm sure you can do a 327 and roots blower within 7k or less. SBC internal parts should be fairly cheap. Find a good block and start from there. The rest of your money can go towards suspension to make the power hook up with the pavement decently.