I think the 6 volts will make it take a very long time to get to full charge if at all but there are plenty more options.
Firstly I have two sets to service with these batteries and I would like to keep them all the same.
As the self discharge of the cells I am using is much better than early NiMh cells I will be making three batteries for this project in order to have a fresh one handy all the time.
The transformers I have are 6 volts so that is what I will be running with.
The cells and the transformer will fit in with space to spare even 8 cells could go in with a little bit of bracket trimming or by laying the transformer in a different position. The pins are tinned copper and can be safely bent. With a doubler I would be tempted by 9.6 volts with 14 volts for charging.
With an external charger I have some larger 2000mah cells left over from a 7.2 volt to 500 volt inverter that I built earlier that works well with an external wall wart that could be pressed into service for these too as they are tagged so no holders will be needed. There will be no space for a charger as they will not fit in with a transformer unless I only go for 7.2 volts with a tight squeeze to get the transformer in. These batteries had a short life due to excessively complicated and unreliable charge management arrangements in there previous life.
We have to remember that these nice old sets need to be running close to 9 volts here with the dismal radio reception in my area. FM is bad enough that we actually need long wave for radio 4
I have rigged an 8 cell battery with PP3 studs and an adapter for charge cycle estimates and will allow this test to to run to completion.