If your old buffer is 2" longer don't use it with the new buffer tube. If the Magpul kit was sealed and not a mixmaster then use the new parts not your old ones.
pawjr74, carbine buffers come in standard, H, H2, H3, 9mm weights. No doubt there are more. They're used with the normal M4 adjustable stocks. Rifle buffers are a different creature and must be used in a rifle receiver extension.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek