Ya can't weld rusty junk. If those pipes are that thin, forget repairing them.
I tig, mig and stick weld steel, aluminum, bronze and do a little stainless steel work. I'm not picking up a propane torch unless I'm soldering copper and even then, I prefer acetylene and silfoss.

I've never had any issue bleeding my diesel, probably because when I'm messing with coolant, I have the front end up on ramps. I fill it up, start it up, watch for the thermostat to open and add more coolant. After a short ride and cool down, I'll top up to the Kuhlwasserstand! ;-)