5150 with avatar 2x12

Hello all, I was told the speaker reps here are very knowledable
and I need some help figuring out my new set up.

First off, I have an old block letter 5150 that I would love to
continue to use with the new avatar 2x12 I ordered last week.
However, the cab's rated at 60w while my head pushes 120w.

I've been told that taking out either the inner or outer pair
of power tubes and halfing the ohms on the amp (so from 8ohm
to 4ohm on the selector) will change the wattage of the 5150 to
60w instead of the original 120w to work with the 60w cab.

With that said, is this completely true, and will the wattage and
overall volume of the amp be the only thing effected by doing so?
I'm aware that getting a second 2x12 would solve this issue as well,
but I'd really like to stick to one cab for now due to money and such.
Any help concerning getting my 5150 to work with the avatar 2x12
would be much appreciated.