bottom fer' sure i this is a serious 6 or 7 second car. the bottom bars of the 4 link are on a push/compression force when you let the rip off the line, thats when they need triangulation from the wishbone. If the chassis does generate any roll rotation the whole deal rotates on the bottom bars and not the top anyways. Some people put the wishbone on the top so it's easier to dump the third member out, this would be ok on a street type ride or anything weaker than a 9 second deal.
the reason i ask is iv got a mate back in NZ with a wild 9.9 tube chassi car (GM thing) with a very cool set up wishbone at the top but most iv seen else wheres have em on the bottom. thanx for your input, i'll be putin mine on the bottom