[Arran]

Leave the 6X5 alone, if they were as bad as the chattering classes think on the barking dog forum all of these sets would not have survived until now. Not only have they survived but there are hundreds of sets, manufactured by Philco, Packard Bell, and other makes that employed them and the vast majority still have their original power transformer. In fact I am working on two right now, they are Canadian built Electrohome sets, one has an issue with an inductor but it is not the power transformer, nor is it the filter choke, it's the antenna coil, probably due to a lightning strike. If a set is designed to use a tube rectifier I would stick with the tube rectifier, ready made SS subs can cause all kinds of problems from the B+ being too high to the fact that they are instant on. You can use a SS rectifier but you usually need to make your own using a current limiter and a dropping resistor, many of the ready made ones are often just straight diodes epoxied in a tube base.
With regard to the dual section filter capacitor, I think that you will have to make your own using individual electrolytics. There are some multi section caps still available but they are expensive and the selection is limited. You can have a go at restuffing the original can, or just install the new ones under the chassis someplace, such as on a terminal strip.
Regards
Arran