Like he said, been covered to death. Ask 10 people that have solved this problem and you will get 7 different opinions that will all conflict each other. However well intended, I would look towards someone who has successfully done this. As with everything on the internet, you have to filter the information. This includes this post as well.
Most people including myself have found the OE-sized radiator to be inadequate with a SBC, whether it be a standard-duty replacement (Radiatorbarn.com, discountradiators.com, CCoT, etc), a high efficiency replacement
http://store.yahoo.com/coolfj40/radfj19thru1.html or a spendy 'this will cool your V8' standard footprint radiator like
http://www.man-a-fre.com/parts_accessories/aluminumradiators.htm
Having owned multiple 40's with V8's, I have tried just about every trick and have absolutely no idea how people living anywhere the ambient temperature is over 75F using the stock radiator with a V8 keep them cool. I guess it depends on how one defines 'cool'.
Factors like your creativity, fabrication/wiring ability and of course budget will all affect the direction you take.
Most will agree you must increase the cooling area of the radiator. This is not accomplished with any of the direct-fit radiators. I am not saying these won't work, just that they are an expensive alternative and many have reported being disappointed in the results. These gain cooling capacity by different materials, depth or number of cores. Now I know AA, MAF and Ron Davis Racing did or still do offer ones using the standard mounting system but these are spendy. Here's a pretty cool kit that addresses all the issues
http://www.man-a-fre.com/parts_accessories/customv8radiator4plusmount.htm
If you have more money than time and ability, I'd go this route.
If it's the other way around you might want to explore using some variation of a generic GM cross-flow design. These are available for usually less then $200 from a number a sources including Summit, Jegs or any of the circle-track suppliers such as:
http://www.daymotorsports.com/product/650/c/14/#
http://www.speedwaymotors.com/xq/aspx/display_id.3541/qx/product.htm
http://www.performancebodies.com/store/dept.asp?dept_id=530
http://www.howeracing.com/Radiators/Index-Radiators-CrossflowChevy.htm
You will have to come up with your own mounting system and shroud as these don't just bolt in.
Good luck and share what you end up with to add to the knowledge base.