Any concerns with the installation, its a Bushmaster 20" v-match setup now. Do i need another barrel extention or do i use the one thats already on the v-match?  I will take it to a gunsmith for a headspace check when I"m finished.

The DPMS barrels I've seen all have the barrel extension installed.  You will need a new forend/handguard to fit the 16" barrel since the 20" barrel probably has the long free float tube on it.  You may want to ask more questions or do a little more research as you get into your project.  Good Luck.

You could pick up a "field" headspace gauge and check it yourself unless the gunsmith is giving you a good deal.