Hi,

I have a very old Colt AR15 Sporter 1 that I bought back in 1979 (has the triangular handguards). I feel like converting it to a carbine. I guess that I can buy a preban upper carbine from bushmaster and a telescoping scope.
From reading the forum, I think that I may need a thingie due to the different size of the Colt and Bushmaster pivot pin.

Is this all that I need? Also, how well will my Colt lower match with the bushmaster upper? The Colt lower is black (http://www.idsi.net/~kinkng/rifle.jpg).

You will need an offset front pivot pin, which Bushmaster or DPMS will sell you cheap.  No way to tell how the receivers will blend until you try it.