[Drake] tube question

[John King]

I know that this has been discussed a million times and I hope asking again doesn't offend anyone.

I understand that Drake's tube of choice was Sylvania brand. My TR3 needs three 12JB6 tubes. The shocker was that when i started scrounging for Sylvania 12JB6 tubes I came up with two "short in height" Sylvania 12JB6 tubes and one "taller" Sylvania 12JB6. My gut feeling is that they should not be mixed due to the different lengths of the elements in the "short" and "taller" tubes. What are your thoughts?

What other brands of 12JB6 tubes have been found to function well in the TR3? Have the tall and short tubes worked  the same? Do you know if they have worked well when mixed in the final of a TR3?

I need 12JB6 tubes, but I need tubes that will work through out the frequency range covered by the TR3. 

The benefit of your actual knowledge and experience will be greatly appreciated. I also still need 12JB6 tubes that will work as Drake final amplifier tubes. 73 and Merry Christmas, John, K5PGW