I'm installing a 2 core aluminum radiator in my xj but it's a manual and does not need the trans cooling part.. my question is can I utilize that side of it and help keep it cooler by filling it with something and sealing it off

 

Im not sure. It might be more stable with coolant in it.

Do some before and after temp comparisons

 

Leave it open to " vent" to atmosphere. It might stay a little cooler that way. If you fill it with something? And cap the ends, there is no room for expansion when whatever you put on there gets hot.

 

Just ignore it. You won't gain anything unless you run a pump and external cooler. In other words, not worth it.