Altec 1567a mod. Reversable

I have a pair of restored Altec 1567a mixers.  I'd like to mod atleast two channels of each into a summing mixer, leaving the other two stock. 

I have four restored Grommes G5M mixers.  I'd like to mod the Altec to take the 600 ohm outputs of the Grommes.  I have four 4665 and atleast four extra 15095 transformers to use.  If I used the 4665 I could toggle switch to change impedances.

Is this feasible?  Am I delusional to try to change from something that works.