They do make the fitting but you do not want to have to pay for it, you can get a close bushing, 1 inch to 3/4 that works nice but you will need to remove the stop in it so the 3/4 pipe will slide all the way through, another thing that I do is to solder scrap pieces of 12 gauge wire loosely to the 3/4 inch pipe, this increases the surface area of the copper exposed to the wood boiler side of the heat exchanger, ( better transfer ) as for the L or M copper, because it is domestic pressurized water it should be L as it is thicker wall