Hi Prad,
I installed a Dewitt radiator on my 67. Your 71 should be same principles. The direct fit should fit with little or no modifications unless you upgrade from 3 core (327 setup) to 4 core (427 setup). Mine just needed some minor cutting of the lips on the lower cushion supports.
In your case, since you need a new radiator support, you can buy either the small block or big block support, cushions, and cushion supports to go with the radiator you want. As mbp mentioned, I would definately call them, they have great customer service and will tell you exactly what you need. If you have a big block, you can also buy with the Spal dual fan setup as these big blocks are prone to over heating when idling in traffic.
I do have a suggestion for you. You mentioned an aluminum radiator support. Not 100% sure if you meant that you are looking for a support made of aluminum. I do NOT recommend that. There is not much stability on the front end of those C2 and C3 corvettes. That radiator support not only supports your rad, but is a major component of the support for the front nose. Unless you are building your car for the track, I would stick to a more solid metal support.
Good Luck
Luke