With the breech block, depress the firing pin extension before pulling up on the breech block and it may come out of the carrier easier. I've never met one I couldn't take out. Anmd some were very tight. The old parts fit easier possibly because they broke in together. Remember these do have some degree of weapon tech fitting required. Checking headspace may be required too. You should be able to take all the folding stock parts off the new lower and put them on your old lower. Just look at them. That would sort out your lower fit problem. I don't know if one's metric and one's inch. That could be the problem too. You will have to look for binding spots in the Carrier to figure out why it won't travel freely in the upper. Some minor stoning my be required. Like you figure, agressive power tools should be avoided.