OK, got the dents in the crossmembers hammered out (more or less). Sorry for the poor quality pics. On LH side where dent was localised, I cut a round hole above it and will put in a plastic plug. On RH side where nearly the whole crossmember was caved in, I ended up cutting a rectangular slot. Left enough metal on the edges so I can pop-rivet a closing plate on top. Will seal it all up with seam sealer then re-cover with deadener pads. For the underside, I’ll scrape off any loose deadener material then slather on a thick layer of brushable deadener and plaster it smooth, then once dry I’ll spray the whole lot with deadener. Should. Look a lot better than before anyway. I also noticed on the RH side where the trans crossmember attaches to the longitudinal rail the two weld nuts and surface of the rail are pushed upwards. Not sure whether I can put a heavy plate on there and use a bolt and the nut to pull it back out, or I could bolt something on there and use my slide hammer. This car had the rubber-isolated crossmember and I’ve seen one side dented in on a car before similar to this - not sure if it gets pushed in due to the torque reaction of the engine, or something else? The newer cars are hard-mounted with an aluminium spacer IIRC.
LH side:
RH side: