That was an interesting situation. The best thing to remember is that when we say "pushing out" the takedown pin, it really doesn't push out of the receiver, but just to its second detent position, which clears the pin from the upper's lug, but not fromthe lower receiver. The best way to do it is to use a punch made of a non-marring substance (delrin, etc..) to just start the pin, going from the left side of the receiver towards the right side of the receiver). tap it lightly to get it started, as it will be tight. Once started, it may still be tight, but use either the same punch, the non-ink end of a Bic pen,etc...to push (don't tap, pound, hammer, or anything else) the pin to its second detent position, which should be where it naturally wants to stop anyway. On a factory assembled rifle, yur upper and lower will be very tight initially, but once it has a few hundred rounds through it, it will still be snug but easier to work with.
Hope that helps. If you have any questions, feel free to give me a call (309 792-5780, 8-4 CST, M-F)and I can walk you through it as well.