[mr48chev]

Most guys now days use an aftermarket MII crossmember with either the stock MII control arms or tube aftermarket control arms. There are dropped spindles to get them lower if needed. Price range is everywhere from the 200 and something fold it and weld it Welders series crossmember through the bolt on units and up through the custom units like what couple of our members sell.
As for the Corvette pieces Most I've seen use the suspension pieces from C-4 series cars.

If you have a really good, straight an solid original frame that isn't rusted, hacked or bent I'd run it. The time to do a frame swap comes when your frame is not in good shape or when you don't have a frame such as after you take a cab off a 1-1/2 ton truck to use for your build.

From experience (not all good) I'd have to say plan everything out before you start. I've got enough parts that I bought and didn't use to probably build two trucks and that doesn't include all the things I picked up for my truck and traded off because I decided not to use them.