Re: easy/safe to weld exhaust pipe with propane torch?
I don't think you will be successful using a regular propane torch even with a Map tank. Map plus Oxygen makes a lot more heat. I attempted a copper pipe with silver solder and within a few minutes several of the joints melted from the heat of the exhaust. I wasted a good bit of money between the copper joints and silver solder with nothing to show for it.
If you need to stay in budget and also want to be sure of success, have the shop do it. If you are willing to invest more for the right stuff then do it yourself, especially if you think you will be doing this again on another build.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...