pre-heating with a torch. Don't get the block red, just hot enough to radiate a pretty good bit of heat to a hand held near the repair.
At trick I learned long ago is to shut off the oxy to the torch and apply a good coating of carbon to the object to be pre-heated and then dial in the oxy and heat. If I remember correctly, carbon burns at 800 degrees and you can tell how quickly it disappears how hot and evenly heated your part is. As the part gets hotter it takes less and less use of the torch to make the carbon film disappear.
I haven't done this in 30 years but I know it worked back then.
