I have a Randall RX412 Cabinet. It is a Mono Cabinet. It says 200 watts continuous, 400 watts program, Impedence 8 OHMS. The Cabinet is a mono.
I have a Line 6 Spider III 150 Watt Head on the way.
My understanding is that the Line 6 head is a stereo head. I've heard that if both outputs aren't used on the head, you can burn it out. I don't know if this is true or not.

What are my options to make this work? Please understand that buying another cabinet is not an option right now.

I've been told I could get somthing called a Y adapter where it would converge the stereo outputs from the head into one mone plug to plug into the cabinet. Is this a good option?

The other option I'm thinking of would be to turn the 4x12 cab into a stereo cabinet but I'm not sure how I would go about this. Would I need two separate Mono Jacks on the Cabinet or do they make a Stereo Jack so I don't need to cut another hole in the cabinet?

Any advice is greatly appreciated.

Thank you.
Supposedly, both channels want to see a load: http://line6.com/support/entry.jspa?externalID=4479&categoryID=117

You could wire your cab for stereo if you connected two speakers together for left, and two together for right. How you connect them makes a difference. It looks like the amp supports a 4 ohm total minimum, which means you can connect your speakers in only one way to stay safe. You'd have to connect two speakers in series + to -, then connect each pair of speakers to either a stereo jack, or separate mono jacks. If memory serves, this type of wiring was covered in this forum more than once, with pictures. Try to do some searching.