If thats your remit...then as Mr Rainbird says....Bruce's shell would be an ideal base.....i'd go square beam and some decent arms.....the major difference between the 2 shells is the rear suspension....bigger suspension platforms....diff cradle and lateral links.....but that in itself spells lots of cash.....the cradle is bossed through the shell into the cage and large turrets are installed....you cant easily convert a grp'a' shell to WRC its a major cut and weld job....but for smoothish tar the later grp'a' cars were fantastic....

pics of a wrc shell interior....note the big turrets....(cant do photshop)