morch_66479 HG = head gasket
I think you probably can do it with the motor in-situ, the biggest challenge will be ensuring no metal swarf gets into the oil system when you tap the block.
You can also remove the sump and weld a return in the side of it, above the baffle line, but the challenge here is that it is very difficult to remove the sump with the motor in-situ. You can do it, generally you have to remove all the bolts, drop it as far as you can, get in from the side with a spanner, disconnect the oil pump, and drop all of it out together. That's not too bad. Putting it back in is another story though, because it is basically impossible to seat the oil pump and its pickup tube correctly while only being able to access it through the gap between the block and the sump. I've done this in a 240 once, the way I did it was I actually cut material off of the internal baffle until I had enough clearance to install the sump with the oil pump already fitted, which is not ideal.
Edit: the other option is you support the engine from above, drop the cross-member, and access it that way, but to me this is basically just as much work as pulling the engine, except that you can do it without needing a crane.
