You may laugh at this suggestion, but it has saved my butt a few times... When you decide on which boiler you are going to buy, Take cardboard and make a stiff box the same shape and size as the boiler will be.. Then using what ever ropes, chains, hoists, wheeled carts, or skids you plan on using, make several trial runs into the house, around the corners, down steps make turns on landings etc..
This 'play-acting' has often shown me flaws in my plans, and I was able to change plans, add reinforcements, pad corners and in general do a better job when the real object the HEAVY object was what I was moving...
You can glue brown corrugated cardboard with Elmers 'white glue' . With a few weights or clamps and some creativity, you can make a very stout box in a few minutes.