It should not be difficult to convert the finals to the 6146 tubes. There
is an article in Hints and Kinks, I recall, on converting another rig using
sweep tubes to 6146s and it would be helpful.

The conversion should be quite straightforward and I would be willing to bet
that it has been done before and information should be on the internet.

In fact, you could probably use one of the 12 volt type 6146 tubes, which
are even cheaper than the 6146 types.

73, Colin K7FM