Let's unscramble this and get you going; you don't need the extension, just forget it. You do need the short vented screw that Robocop posted. Go ahead and get you one of those.
Next is your buffer, it's fully .250" too long, and needs to undergo the following changes:
#1. Drive the roll pin out.
#2. Remove the rear plug and bag it along with the guts of the buffer, they will dump right out.
#3. Cut 1/4" off the rear of the buffer tube as cleanly as you possibly can, removing any burrs that result from doing so.
#4. Measure the distance between the shoulder on the rear plug and the through-hole for the roll pin.
#5. Use that measurement to re-drill the roll-pin hole in the buffer body. Roll pins are standard imperial sizes so simply use the appropriate drill bit, mine was 1/16".
#6. Re-assemble the buffer minus the aluminum spacer that is rearmost in the buffer assembly. It's unnecessary and won't allow you to reassemble the buffer unless you cut it by half an inch as well.
That's it! If you're not comfortable doing this, you can ship it to me and I'll fix it free of charge.