So, yesterday it happened: I cut out the first piece of sheet metal to be replaced with new.
Still needs more rectangular pipe and steel for brackets, so I started with tacking the sub frame legs to the subframe, in order to release the welding clamps. Then the rest of the evening was spend with aligning and clamping the new bottom of the cowl to the existing, cutting through both of them at once with an airdriven saw and cut the old piece off (forgot to take pictures underway, this is taken afterwards).
Unfortunately the inner profile was very rusted, so I'll have to grind it down and give it some zink spray, before the new piece could be welded on.
So, now I know, how to do it on the drivers side as well - and if there is still no rectangular pipe and bracket steel next week, I'll grind down and spray, and start on the drivers side.