I removed the radiator on my '78 P30 to have it re-cored etc. When removing the hard lines from the transmission to the integrated cooler in the bottom tank, one of the lines sheared off right at the fitting, so I will need to replace at least one of them. I'm sure in a pinch I could use some rubber lines and hose clamps but the only semi-official advice I could read on this was in the Chevrolet Motorhome Chassis Service Guide on line 205.

Is anyone familiar with this standard/process? Any tops for how I can safely replace these lines per GM's requirements?

Very much like brake lines. Cut it back to where the line is good. Flare the end and ad a new section. You can also build a new line from the transmission. If the fitting is damaged at the radiator the shop doing the re-core should replace both fittings.

You could also install a transmission cooler in front or behind the radiator. This depends on where you have space.

A rubber extension will eventually deteriorate from the oil. Seek out oil resistant rubber.